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. 
So I know that was our original intention, but looking at a lot of the issues that have come in I feel that adoption of the new parameters is slower than we originally anticipated. I think we should hold off on deprecating things until a later release. Just my opinion though, I can modify this pull request if we choose to remove the deprecated parameters.