Adding debugger from oahu

Allows for per-trigger-definition debugging. Simply add
debug_level=1 to your trigger definition.

debug_level=2 gives detailed explainations of why your
stream didn't fire or trigger.

This way, you can leave the overall debug level at INFO
and still get details on the stream you're working on.

Yagi worker now uses idle() callback for debug updates.

and minor cleanups for pep8/HACKING

Change-Id: Id0af7a0adbcc47335ad250130958932c708b5c18