Silence C extension RuntimeWarning PYTHON-569

When encoding a document in C while running in a python sub interpreter
(e.g. mod_wsgi spawning sub interpreters) PyMongo would often have
to reload its cache of pure python types - ObjectId, Timestamp,
UUID, etc. - raising RuntimeWarning in the process. The reason this was
necessary is described in the mod_wsgi documentation here:

https://code.google.com/p/modwsgi/wiki/ApplicationIssues#Multiple_Python_Sub_Interpreters

With workarounds documented here:

http://api.mongodb.org/python/2.6.2/faq.html#does-pymongo-work-with-mod-wsgi

With this commit PyMongo will no longer use cached pure python types
when running in a sub interpreter. Instead it will look up pure python
types as needed (primarily when decoding BSON). This eliminates the
problem described in the mod_wsgi docs and eliminates the need for the
Runtime warning.