Gridfs alternate host and mongo hardening

Added mongo connection proxy from https://github.com/arngarden/MongoDBProxy to autoreconnect when the mongo connection is interrupted.  Added default socket timeouts to our mongo connection, as otherwise when a replica set member goes down we never get a chance to reconnect.

Excluded girder/lib/* from static analysis and from coverage.  The lib directory should only be used for external modules we can't install via pip.

Added support for GridFS assetstores to use a different mongo database than girder.  This adds a guard against connecting to an offline database.

Added a python file to create a mongo replica set for testing, and added replica set testing.  This includes testing reconnecting to a mongo server when it is disconnected.

This addresses issues #438 and #594.