Optionally include platform-specific end reason codes

Some applications require more detailed end reasons such as circuit congestion, etc. [Asterisk](http://www.voip-info.org/wiki/view/Asterisk+variable+hangupcause) and [FreeSWITCH](http://wiki.freeswitch.org/wiki/Hangup_causes) provide their own enumerations, while other platforms (eg PRISM) might expose SIP codes or something else entirely. This patch allows for passing those specific codes up to the Rayo client after categorisation as one of the more general Rayo end reasons.

@crienzo, @bklang and @loopingrage: I'd like your input on this, please.