Automatic connection recovery.

New configuration parameter: 'connection_recovery_min_interval' (seconds)
New configuration parameter: 'connection_recovery_max_interval' (seconds)

When all outbound proxies get unreachable, User Agent attemps to reconnect applying an
exponential backoff algorithm, which takes the minimum and maximum reconnection interval
values from those new configuration parameters.
Make preloaded Route header optional.

New configuration parameter 'use_preloaded_route'

Preload Route header with the outbound proxy only if this parameter is set to *true*
Enhance 'boolean' comparison
Add turn configuration parameters for RTCPeerConnection

* new configuration parameters: turn_server, turn_username, turn_password
Update the compiled  parser grammar

* new stun_URI rule
* new turn_URI rule
Fix configuration load for parameters accepting (0, false, null) values.
Emit 'connected' event as soon as the transport is connected
Fix extra comma in Digest Authentication Response

when no 'qop' was present
Merge branch 'master' of https://github.com/versatica/JsSIP
Fix the election of Authorization and Proxy-Authorization
Merge branch 'master' of https://github.com/versatica/JsSIP
Fix new dialog creation check
Cosmetic change
Enhance RTCPeerconnection SDP error handling. Thanks @ibc for reporting.
Merge branch 'master' of https://github.com/versatica/JsSIP
Remove unused variables, as stated by grunt.