Session: on re-INVITE, send 488 instead of calling this.receiveReinvite

See https://github.com/onsip/SIP.js/issues/19#issuecomment-43887857

I tested this against a conference call using Freeswitch 1.5.11 installed as [described on sipjs.com](http://sipjs.com/guides/server-configuration/freeswitch/), received and rejected the re-invites, and was able to remain in the conference for several multiples of the `Session-Expires` header's value afterwards. I didn't receive any more re-invites, so I think rejecting the first one successfully tells Freeswitch we don't currently support session timers.

This also prevents non-timer renegotiation (example: hold), which seems ok since the browsers don't currently have general support for it. See https://github.com/onsip/SIP.js/issues/19#issuecomment-43680028

fixes #19