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-virtual-functions.py b/example/example-virtual-functions.py
new file mode 100644
index 0000000..9580019
--- /dev/null
+++ b/example/example-virtual-functions.py
@@ -0,0 +1,36 @@
+#!/usr/bin/env python
+from __future__ import print_function
+import sys
+sys.path.append('.')
+
+from example import ExampleVirt, runExampleVirt, runExampleVirtVirtual, runExampleVirtBool
+
+
+class ExtendedExampleVirt(ExampleVirt):
+    def __init__(self, state):
+        super(ExtendedExampleVirt, self).__init__(state + 1)
+        self.data = "Hello world"
+
+    def run(self, value):
+        print('ExtendedExampleVirt::run(%i), calling parent..' % value)
+        return super(ExtendedExampleVirt, self).run(value + 1)
+
+    def run_bool(self):
+        print('ExtendedExampleVirt::run_bool()')
+        return False
+
+    def pure_virtual(self):
+        print('ExtendedExampleVirt::pure_virtual(): %s' % self.data)
+
+
+ex12 = ExampleVirt(10)
+print(runExampleVirt(ex12, 20))
+try:
+    runExampleVirtVirtual(ex12)
+except Exception as e:
+    print("Caught expected exception: " + str(e))
+
+ex12p = ExtendedExampleVirt(10)
+print(runExampleVirt(ex12p, 20))
+print(runExampleVirtBool(ex12p))
+runExampleVirtVirtual(ex12p)