Initial support for Firefox OS

I'm working on adding phonegap developer app to firefox OS. I have it working pretty well, I wrote a blog post about it [on hacks.mozilla with a screencast of it running](https://hacks.mozilla.org/2014/09/phonegap-developer-app-preview-for-firefox-os/).

This change was required to add `phongep serve` support for firefoxos.

As with https://github.com/phonegap/phonegap-app-developer/pull/208, I don't think this PR is ready to merge until PhoneGap gets all 3.6 goodies, but would truly appreciate any feedback.