Support for Python 3 in libclang python bindings

Summary:
Python bindings tests now pass in Python 3.

map in Python 3 is lazily evaluated so the method by which functions are registered needed updating.

Strings are unicode in Python 3 not UTF-8, I've tried to create an new c_types-like class (c_string_p) to automate the conversion.

String conversions made explicit where required.

Reviewers: eliben, nemanjai, skalinichev, compnerd

Subscribers: mgorny, cfe-commits

Differential Revision: https://reviews.llvm.org/D26082

llvm-svn: 285909