Make contributing easier

This pull request includes a series of changes that I believe make development easier (especially for new contributors).  I've separated these changes into multiple commits so I can easily modify each change (if necessary) based on your feedback.

Changes:

* Allow `python setup.py test` to work for easy package testing
* Test all supported Django versions explicitly on Travis CI
* Test code coverage on Travis CI using [Coveralls][] and add a coverage badge to README
* Test against Python 3.2 and 3.3 (they were supported without any code changes)
* Add [tox][] support for testing against multiple Python/Django versions and measuring code coverage locally
* Add `Makefile` demonstrating usage of `tox`
* Add more globs to `.gitignore` file
* Remove requirements.txt file (`install_requires` in setup.py replaces this)

[coveralls]: http://coveralls.io/
[tox]: http://testrun.org/tox/