Add get_player_state method to retrieve the state/status/speed of the speaker

Currently there is no reliable way to find out if the speaker is playing or paused/stopped.
SoCo uses the `GetTransportInfo` call which I've implemented as `get_player_state`:

    irb(main):004:0> state = speaker.get_player_state
    D, [2014-01-20T20:51:42.982358 #29285] DEBUG -- : HTTPI POST request to 192.168.178.113 (httpclient)
    => {:status=>"OK", :state=>"PAUSED_PLAYBACK", :speed=>nil}