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. 
I know legacy support was mentioned in [the original issue](https://github.com/onsip/SIP.js/issues/27), but since this is now part of [the v0.6.0 milestone](https://github.com/onsip/SIP.js/issues?milestone=1&state=open), I think we should drop it and require the constants to be used.