TST: Set up Travis-CI to auto build pandas' PRs
@wesm - can you do this - http://about.travis-ci.org/blog/2012-09-04-pull-requests-just-got-even-more-awesome/ ?

Looks like you just have to tweak a setting and then Travis will build every PR automatically. I don't have admin rights to pydata so I can't change it.

And then we wouldn't have to tell everyone to set up Travis. That would be _great_
