Google cast support

[Ichabond]

With https://github.com/dz0ny/leapcast available, it would be a nifty feature to add to imp. Issue being NAT traversal, same as with Apple Airplay. But still, would be nice, I think.

[Ichabond]

With Chromecast and airplay now both being available on the system, having both integrated as "streams" in the queue would prevent double or triple queueing of items.

Example of usage: imp add chromecast would put the chromecast server in the queue, and once it's its turn, it would act as an infinite stream. Same applies to airport.