Provide more control over automatic generation of docstrings (#486)

Added the docstring_options class, which gives global control over the generation of docstrings and function signatures.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 48d3cc7..6aa2bea 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -160,6 +160,7 @@
   include/pybind11/common.h
   include/pybind11/complex.h
   include/pybind11/descr.h
+  include/pybind11/options.h
   include/pybind11/eigen.h
   include/pybind11/eval.h
   include/pybind11/functional.h