Rename examples files, as per #288

This renames example files from `exampleN` to `example-description`.

Specifically, the following renaming is applied:

example1 -> example-methods-and-attributes
example2 -> example-python-types
example3 -> example-operator-overloading
example4 -> example-constants-and-functions
example5 -> example-callbacks (*)
example6 -> example-sequence-and-iterators
example7 -> example-buffers
example8 -> example-custom-ref-counting
example9 -> example-modules
example10 -> example-numpy-vectorize
example11 -> example-arg-keywords-and-defaults
example12 -> example-virtual-functions
example13 -> example-keep-alive
example14 -> example-opaque-types
example15 -> example-pickling
example16 -> example-inheritance
example17 -> example-stl-binders
example18 -> example-eval
example19 -> example-custom-exceptions

* the inheritance parts of example5 are moved into example-inheritance
(previously example16), and the remainder is left as example-callbacks.

This commit also renames the internal variables ("Example1",
"Example2", "Example4", etc.) into non-numeric names ("ExampleMandA",
"ExamplePythonTypes", "ExampleWithEnum", etc.) to correspond to the
file renaming.

The order of tests is preserved, but this can easily be changed if
there is some more natural ordering by updating the list in
examples/CMakeLists.txt.
diff --git a/example/example.cpp b/example/example.cpp
index ad37273..e62fde7 100644
--- a/example/example.cpp
+++ b/example/example.cpp
@@ -9,25 +9,25 @@
 
 #include "example.h"
 
-void init_ex1(py::module &);
-void init_ex2(py::module &);
-void init_ex3(py::module &);
-void init_ex4(py::module &);
-void init_ex5(py::module &);
-void init_ex6(py::module &);
-void init_ex7(py::module &);
-void init_ex8(py::module &);
-void init_ex9(py::module &);
-void init_ex10(py::module &);
-void init_ex11(py::module &);
-void init_ex12(py::module &);
-void init_ex13(py::module &);
-void init_ex14(py::module &);
-void init_ex15(py::module &);
-void init_ex16(py::module &);
-void init_ex17(py::module &);
-void init_ex18(py::module &);
-void init_ex19(py::module &);
+void init_ex_methods_and_attributes(py::module &);
+void init_ex_python_types(py::module &);
+void init_ex_operator_overloading(py::module &);
+void init_ex_constants_and_functions(py::module &);
+void init_ex_callbacks(py::module &);
+void init_ex_sequences_and_iterators(py::module &);
+void init_ex_buffers(py::module &);
+void init_ex_smart_ptr(py::module &);
+void init_ex_modules(py::module &);
+void init_ex_numpy_vectorize(py::module &);
+void init_ex_arg_keywords_and_defaults(py::module &);
+void init_ex_virtual_functions(py::module &);
+void init_ex_keep_alive(py::module &);
+void init_ex_opaque_types(py::module &);
+void init_ex_pickling(py::module &);
+void init_ex_inheritance(py::module &);
+void init_ex_stl_binder_vector(py::module &);
+void init_ex_eval(py::module &);
+void init_ex_custom_exceptions(py::module &);
 void init_issues(py::module &);
 
 #if defined(PYBIND11_TEST_EIGEN)
@@ -37,25 +37,25 @@
 PYBIND11_PLUGIN(example) {
     py::module m("example", "pybind example plugin");
 
-    init_ex1(m);
-    init_ex2(m);
-    init_ex3(m);
-    init_ex4(m);
-    init_ex5(m);
-    init_ex6(m);
-    init_ex7(m);
-    init_ex8(m);
-    init_ex9(m);
-    init_ex10(m);
-    init_ex11(m);
-    init_ex12(m);
-    init_ex13(m);
-    init_ex14(m);
-    init_ex15(m);
-    init_ex16(m);
-    init_ex17(m);
-    init_ex18(m);
-    init_ex19(m);
+    init_ex_methods_and_attributes(m);
+    init_ex_python_types(m);
+    init_ex_operator_overloading(m);
+    init_ex_constants_and_functions(m);
+    init_ex_callbacks(m);
+    init_ex_sequences_and_iterators(m);
+    init_ex_buffers(m);
+    init_ex_smart_ptr(m);
+    init_ex_modules(m);
+    init_ex_numpy_vectorize(m);
+    init_ex_arg_keywords_and_defaults(m);
+    init_ex_virtual_functions(m);
+    init_ex_keep_alive(m);
+    init_ex_opaque_types(m);
+    init_ex_pickling(m);
+    init_ex_inheritance(m);
+    init_ex_stl_binder_vector(m);
+    init_ex_eval(m);
+    init_ex_custom_exceptions(m);
     init_issues(m);
 
     #if defined(PYBIND11_TEST_EIGEN)