gevent based pool

This patch permits to select a different connection pool class.
Included there is a wrap to the gevent psycopg2 poll suitable to be used
in a events based environment (like gunicorn).
added convenience method to close all connections
warmup function
explicitly returns the connection to the pool
Merge branch 'master' into gevent_pool

Conflicts:
	dbpool/db/backends/postgresql_psycopg2/base.py