Refactored Session

Use the request call_id property

Fix typo

Put MediaSession into Session

Delete Session 'connecting' and Message 'sending' redundant events

Add custom empty 'data' object to Message

Rename Session by RTCSession and MediaSession by RTCMediaHandler

Update the RTCSession LOG_PREFIX

Add dialog error handling

Renamed Session.js by RTCSession.js

Delete redundant line

Add some extra comments