Removed del db_cache
Merge branch 'master' of https://github.com/fiorix/mongo-async-python-driver

Conflicts:
	txmongo/collection.py
Revert "Fix authentication issue"

This reverts commit 7f0bf7ac0ca81793f02f68ad38660361b39b775a.
moved import to top
fixed tests, auth still broken
pep 8
fixed auth issues
ConnectionPool accepts a URI that includes a username and password.  This fixes authentication for callers under these conditions.
 - Original code successfully set credentials but generated an exception.  This fixes that exception by limiting the auth call to the critical cred_cache setting.