Attended Transfer: UA config option 'replaces' defaults to UNSUPPORTED

This way, applications unprepared for replacement won't be surprised by it.

https://github.com/onsip/SIP.js/pull/136#issuecomment-67249445
Attended Transfer: emit 'replace' on the replacee, provide followReplaces helper

https://github.com/onsip/SIP.js/pull/136#issuecomment-67249445