Continuous integration for asteval on travis-ci

https://travis-ci.org is a great free continuous integration service that integrates well with github.
If you add the attached `.travis.yml` file and enable Travis under Admin -> Service Hooks, travis-ci will test each commit to asteval master and report the results via email. It also automatically tests any pull requests.

At the moment two tests are run, one for Python 2.7 and one for Python 3.2. It is possible to add tests for more Python / numpy versions if you want.

May I suggest the following changes?

1. Declare the optional numpy dependency in setup.py and the README.
2. Rename the `lib` folder to `asteval`
3. Write the unit tests so that numpy actually is optional.

Basically this will give a more standard python package and it will be possible to simply clone the asteval repo and simply say `nosetests` and everything will work.