feat(testing): include iOS and Android Browser in BrowserStack tests

We currently have no coverage for mobile browsers, including tablets. This updated config will include
testing for iOS 6.0 and 7.0 (iPad and iPhone) and Android Browser (v 4.2, Nexus 4 and 7). As far as I
can tell, there is no additional cost and is part of the monthly membership.

SauceLabs doesn't currently support iOS & Android for Selenium, so I excluded them from this PR. https://saucelabs.com/platforms/selenium

All available BrowserStack testing environments can be found here (accurate as of March 6, 2014): https://gist.githubusercontent.com/3lux/9393261/raw/70302480ed602cbb4c9bfdf50bb3293ca7ea6ee2/gistfile1.json