Add Travis CI integration

Hello,
I just came across this nice project, and tried to make it integrated with Travis CI.

Few notes:

* To make CI builds possible, we need to set Xcode scheme shared instead of auto-generated
* Currently there's a bug in `xctool` so we need to install from git `HEAD` (ref: facebook/xctool#273)
* ...and don't forget to set up this project on http://travis-ci.org/ to start building.

Hope you will like it! :smile: