Add get_absolute_url method to all models

The model instances should be able to tell which endpoint contains
its details.
Add a helper mixin for view test cases

The mixin defines some helper methods which are useful when testing
views (REST endpoints).
Create a test package for the chat app

All tests will be contained within the test package, instead of a
giant test.py module.
Add tests for the MemberList endpoint
Add tests for the message list endpoint
Refactor development requirements file

Instead of having the development requirements file list all the
packages which are required in production, adapt the script to install
both packages from ``requirements.txt`` as well as
``requirements-dev.txt``.  That way, the development requirements file
can be stated only in terms of *extra* packages which are useful
for a development environment.