[reSIProcate] Added SigComp Hooks

[Adam Roach <adam@xxxxxxxxxxx>]

I have merged a set of changes into the resip stack (as of this morning) 
which allow it to use the Open SigComp stack to send and receive 
messages compressed with SigComp.

See http://wiki.resiprocate.org/wiki/index.php?title=SigComp_Support for 
a short write up on how to use this new feature.

/a