FirefoxOS support

Thought I'd merge into master the FirefoxOS support I hacked into Appium at GTAC 2013. (See [this video](http://www.youtube.com/watch?list=SPSIUOFhnxEiCODb8XQB-RUQ0RGNZ2yW7d&v=1J0aXDbjiUE&feature=player_detailpage#t=2022s) for the backstory)