[cindex.py] Make the use of a compatibilty check explicit

At the moment, we implictly check compatibility between the python
bindings and libclang, as the python bindings will fail to load in
case a method we use in libclang is not available.

This patch makes the use of this compatibility check explicit and introduces a
flag to optionally disable the check. This will allow us to further harden the
compatibility check, but it also gives the user the possibility to disable the
compatibility check to evaluate compatibility with older libclang versions.

I added documentation that makes clear the python bindings are only tested
with the libclang version they have been shipped with.

llvm-svn: 163238