Remove assert(is_request)

Assert causes crash when we try to handle reply
for M7, M8 and M9 commands that are sent from optional senders.
Optional handlers CanHandle is called and if handler is Request
handler assert is triggered.
Implement handling of play / pause / teardown

This patch adds possibility to control playback
state of MiracGstSink and uses that functionality
in sink state machine.
Merge pull request #26 from alexshalamov/handle_pause_play_teardown

Handle pause play teardown