Added Android Phone as a default device

Hi! I've used this modification in a personal project and maybe you would like to introduce it into the gem.

I think that it is useful to include the android phone as a default device because of the same reason that we have iphone and ipad as two different devices. I would have included android_tablet too, but I can't find the way to do the detection with a simple regexp and also didn't want to do a complicated stuff into the code.

Thanks for your time :)