Automate unit tests with travis

This is a [working example](https://travis-ci.org/adam-lynch/bootstrap-datepicker) of Travis-CI running the unit tests from [my branch](https://github.com/adam-lynch/bootstrap-datepicker/tree/automate-unit-tests-with-travis).

Some further setup needs to be done to get this to work. First, as the owner of the project, you've to create a Travis account (if you don't have one, you just click log in with GitHub and it's done), then go to the project's settings and enable the web hook to Travis (simple to do).
You might have to go to your Travis account and enable this project but probably not.

If this pull request is merged before that setup is done, then the status image will display "unknown".