Add $.browser.mobile and $.browser.desktop

In a bit of hurry so I can't write a longer description right now or run tests. I will write more in a comment later. Basically, this adds `$.browser.mobile` and `$.browser.desktop` options to check if the user is running a mobile or desktop browser (as detected by the platform). I also added examples in the Readme and tests. I cannot test the CoffeeScript (do not have a way to), so I hope it works. :sweat_smile: