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 
But then we might have `iOSSimulator` - do we shorten it to `iSimulator` too? :-)
That's why underscored was easier in this context (`firefox_os_simulator`).

I'm ambivalent, but I just don't want to close doors too early by limiting ourselves early on.