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. 
What if we change the constants to be the corresponding strings instead of 0, 1, 2? Anything gusing constants should keep working, and anything using the strings should start matching up. Then the difference just becomes documentation.