Adds configuration instructions to installation

Assumes virtualenv as target
Use tox with travis

Simpler matrix configuration for testing multiple Python versions (and
interpreters) as well as Django versions.

Removes Django from test requirements file so that each tox environment
is configured without a default, unless someone adds a minimum version
to setup.py. Also includes pypy and Jython environments although they
are not added to the default list of tox test environments.
Add more default files
Mine