Make a chatroom name unique - fixes #2
Add a timestamp field to the Message model

The timestamp is automatically assigned at the moment of creation
of the message. It is represented as a Month-based ISO8601
date-time format when sending the Message resource representation
in JSON.

Closes #1
Add a mixin for filtering view sets
Filter members based on the lrz_id field

The filtering mechanism mixes in the previously implemented
FilterMixin and filters only based on the lrz_id field.

Closes #4
Filter chat rooms based on the name

The chat room filtering is implemented by mixing in the FilterMixin
and specifying that it should be filtered only based on the name.

Closes #5