Add travis integration

- Add basic Travis CI integration to *Arpeggio* project. The Travis CI will simply install *Arpeggio* using `setup.py` script, and run all unit tests in folder `tests/unit`. This will run tests for both Python 3.3.x and Python 2.7.x (this can be changed as necessary). 

TODO: Since I'm not the repository owner, the following things need to be done by repo owner:
1. Read http://docs.travis-ci.com/user/getting-started/ on how authorize Travis CI to access Github account.
2. Optionally add a status image to README.md (http://docs.travis-ci.com/user/status-images/). Note there is Markdown syntax available, that works on most sites.
3. Tweak mailing pattern. By default Travis CI sends automated mails when builds are broken and when they are repaired.