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. 
We could do that, but I feel that it is a stop gap. I feel like most of the advantage of using constants is that we have control over what the value actually is and we can change it at will. If our constants are locked into values to support backwards compatibility it could cause bigger problems moving forward.