Browser versions

Its not the prettiest, but it allows users to specify platform and version when using a remote-url, so

`salad --browser firefox-15-Windows_2012 --remote-url http://somegridprovider/wd/hub salad/features`

Will launch a Firefox 15 instance (the default is 18) on Windows 8 (the default is XP aka Windows 2003) if your grid provider has it available.

You *might* need bleeding edge splinter for this to work. I caught a bug in my initial remote-driver implementation but fortunately it looks like someone fixed it recently.