[cindex.py] Allow to configure the path of libclang

By calling cindex.Config.set_library_path(path) or
cindex.Config.set_library_file(file) it is possible to specify from where
we load libclang. This fixes an open FIXME.

We also point the user to these functions, in case libclang can not be
loaded sucessfully.

llvm-svn: 163121