Logging Improvements

Logging functionality has been completely refactored.

Now Cocaine uses the following Blackhole features:
 - Scoped attributes guard for keeping common attributes while some object lives (see 'bootstrap' section, for example).
 - Logger wrapper for source attribute keeping.

Also attributes now used more widely.
This comment is misleading now. Also, do we plan to use BH in Repo for logging? Or do you plan to use Repo in BH for plugins?