Add OS information for Firefox Mobile on Firefox OS. Closes #27.

Originally I wanted to just add `{ 'label': 'Firefox OS', 'pattern': 'blah'}` to the detectable OSes but `getOS` adds a `\b` before the pattern so it wouldn't match the opening paren (which disambiguates between Firefox for Android, and other imposters with similar UA strings). So this patch just special-cases the detection.