Reworded a few things for consistency- should fix issue #13

I also added an `usage.js` example to make sure the code kept working - this undoubtedly belongs to functional tests, but let's have it here for now. README.md has also been updated to reflect the new options and also enumerate them (which would fix issue #9)

r? @tofumatt 
Could this be `firefoxSimulator` without being confusing and still clear it's FxOS? It's just a bit hard to read with the string of capital letters.

If not it's okay, `firefoxOSSimulator` is better than `firefoxOsSimulator` or something. `firefoxSimulator` just seems a bit easier to read/type.