Fix #54. Allow configuration.uri username start with 'sip'
Fix access to a parsed 'To' header uri host
Allow non array value for 'stun_servers' and 'turn_servers'

...when specifying a single server.
Rename 'outbound_proxy_set' parameter by 'ws_servers'
Fix #26. Fire 'unregistered' event correctly.

Fire the event in any of the following cases:

- SIP response to un-REGISTER request
- un-REGISTER request transaction timeout
- Transport error during un-REGISTER request
- re-REGISTER failure
Fix the order of fired events for registration failure

- Fire first the 'registrationFailed' event
- If the UA is registered, fire the 'unregistered' event
Comsetic. Enhance Utils.sipErrorCause method.
Fix the way stun servers configuration is generated.
Change API methods to make use of generic 'options' argument
Add DTMF feature
Rename 'mediaType' by 'mediaTypes'
Merge remote-tracking branch 'upstream/master'

Conflicts:
	src/MediaSession.js
	src/Session.js