Tox and Travis configuration improvements

This builds on pull request #1.  I've made this a separate pull request because #1 can stand on its own also.

Changes made:
* Fixed Python 2.6 incompatibility (no dict comprehensions in 2.6)
* Use `python setup.py test` in Tox and Travis instead of using `requirements.txt` file and `runtest.py` directly.
* Test both Django 1.4 and Django 1.5 in Tox and Travis


Some more suggestions:

1. I noticed you haven't registered this repository on Travis yet.  You might want to do that so CI tests actually run.

2. Since you're already using Travis and you're generating coverage reports in your tests you could also use [coveralls](https://coveralls.io/) also if you wanted to display or record your test coverage.

3. At this point I would either:
* delete the requirements.txt file and remove references to it, instead recommending usage of `python setup.py test`
* Do something like this in the setup.py file to avoid duplication:

        install_requires=open('install_requirements.txt').read().split('\n')

    and use `-r install_requirements.txt` and `-r dev_requirements.txt` in your `requirements.txt` file if you want to keep a single file for all requirements