Discover and support ChromeCast devices

[auchenberg]

Discover ChromeCast devices using MDNS, and make them appear in the list, if port 9222 is pingable.

Chrome feature request: https://code.google.com/p/chromium/issues/detail?id=444884

[auchenberg]

Chrome has moved forward with this: https://chromium.googlesource.com/chromium/src.git/+/a0e7c26be887f4422707e7b6b674cee67973d6f5