[Reviewer: Alex] Beefing up debug logging, speculative bugfix

Alex,

I beefed up the debug logging in the SDP stack recently in order to track down a bug - can you review and merge it?

There's a speculative bugfix (which didn't actually affect the issue I was seeing) around line 124 of peer.py - the code used to loop over an empty dictionary to work out which Diameter applications the peer supported, rather than looping over the list of applications from the message and inserting them in the empty dictionary.