Allow constants to be used to set rel100 options

Resolves #27.

On a side note, I think we are slightly out of compliance with [RFC 3262](http://www.ietf.org/rfc/rfc3262.txt). I want to re-read the RFC and I will create a separate pull request to address those issues. 
Hmm yeah, in that case we should log warnings if the old values are used. There's also a [couple](https://github.com/egreenmachine/SIP.js/blob/issue-27/src/UA.js#L895) of [places](https://github.com/egreenmachine/SIP.js/blob/issue-27/src/UA.js#L937) still using the old values. Looks good to me otherwise :+1: