Add a simple tox.ini for Astropy testing Python 2.6, 2.7, 3.2, and 3.3

This adds yet another way to run the Astropy tests on multiple configurations.  Though this is more for the use of individuals who would like to run the tests manually than for CI systems.

For those who are unaware of it [tox](http://pypi.python.org/pypi/tox) is a Python-specific program which, when run, creates virtualenvs for any number of configurations, installs your project into it, and runs any tests on it you wish to perform.

The virtualenvs are stored in a .tox directory in your project root, so the virtualenvs don't have to be recreated every time the tests are run (which is especially good when you have requirements).

For now this tox.ini is very simple.  It just tests 4 versions of Python with whatever the latest Numpy release is.  Hypothetically this can be expanded to include any other number of combinations.  It does take a while for the tests to run on all platforms so when I use it I usually start the tests and go off to do something else for a while.  But it's still often quicker than waiting for an automated build.