Add max_load_factor getter and setter for std::unordered_map.

This might be useful for controlling memory usage at the expense of collision probability.
Merge pull request #334 from maciejkula/add_unordered_map_max_load_factor

Add max_load_factor getter and setter for std::unordered_map.
reference naming conflict resolution from C++ wrapping doc, show how to wrap C++ objects in cdef class attributes
Merge branch 'master' of https://github.com/cython/cython
Added tests, adjusted warning message, and fixed the line in PyrexTypes.