add logging during unpacking, verifying

Signed-off-by: Tal Shprecher <tshprecher@twitter.com>
log using slf4j instead of util-logging
Add more logging details on oauth1 verify errors, change all logging
from info to debug, remove slf1j-simple from pom.xml
change the loggers to be static fields
make loggers private in the standard unpacker/verifier companion objects
Merge branch 'master' into logging

Conflicts:
	src/main/scala/com/twitter/joauth/Unpacker.scala
Merge pull request #8 from twitter/logging

add logging during unpacking, verifying