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. 
It looks like it is only one place. The second place that you mention is replacing the deprecated reliable parameter with rel100. I fixed the first place though. I also added a deprecation warning about setting rel100 with a string instead of using the constants. 

It is a shame we did not think of using constants earlier, we could have originally set it up so that rel100 only took constants.