Allow safe use of gevent with django ORM and connection pools

When using the django ORM in combination with a greenlet safe connection pool (like django_db_geventpool) it is necessary to control monkey patching of the thread functions (if they are monkey patched django will handle connections at the greenlet level which will block any gains from connection sharing and leave a bunch of open connections for the db server to timeout)

This PR introduces an environment variable which can be used to disable monkey patching of the threading library.  Additionally some basic documentation for gevent (based on the eventlet one) is included