Added Gitter badge
Merge pull request #57 from gitter-badger/gitter-badge

Add a Gitter chat badge to README.md
adding travis webhook for gitter.

trello: https://trello.com/c/Xb9YOwes/671-gitter
new model caching: fixing `delete_cache` methods.

Trello:
https://trello.com/c/V6polddr/660-neues-konzept-fur-model-caching
applying new model caching mechanism to UserGroupMemberships.

Trello:
https://trello.com/c/V6polddr/660-neues-konzept-fur-model-caching
including gitter.im channel in maintenance info box.