Fix test_socket_reclamation in Gevent 1.0, PYTHON-521
Added Manga to list of Python-based tools.
Fix doc compile warnings.
Fix null pointer when decoding invalid DBRef PYTHON-532
Fix import when _cbson unavailable PYTHON-532

This ensures that we don't import _cmessage in
an incomplete state or segfault if _cbson isn't
available. If _cbson.so has been removed we fall
back to pure python.
Changelog for 2.5.2.
Further DBRef cleanup.
Fix parameter hiding.
Clarify network_timeout and commands PYTHON-495
Fix up REType reload.