Initial commit for MSC2746: Improved VoIP Signalling

- Added version check
- Added 'invitee' to m.call.invite
- Added 'party_id' to all VoIP events
- Added new events
  - m.call.select_answer
  - m.call.reject
  - m.call.negotiate
- Added reason codes to m.call.hangup
- Added custom struct for session description objects