Travis-CI support

Hello!

Because for my last pull request I needed to check that PLY still passed tests for Python 2 and Python 3, I decided to add Travis-CI support. With this addition, it will be tested against 2.6, 2.7, 3.2, and 3.3 with every push, and you will also be able to see the results of the tests on pull requests.

Here are the build results: https://travis-ci.org/kennknowles/ply

![Build status](https://travis-ci.org/kennknowles/ply.png) <-- at the time I am submitting this pull request, this says "Build failing" because it passes for 2.6, 2.7, and 3.2, but not for 3.3

Once merged, you would want to enabled Travis-CI for your fork (and possibly link to the analogous image in the README, which you'll need to rename to README.md for github to render)

- Kenn