Support for Firefox OS

https://issues.jboss.org/browse/AGCORDOVA-21

This comes from https://github.com/aerogear/aerogear-pushplugin-cordova/tree/firefoxos branch.

I've fixed that branch and rebased on top of the `master`.

@sebastienblanc @lholmquist  @edewit  would you give this a spin?