Make DEBUG-level logs consummable.

Removes two things that caused issues with the debug logs:

(a) Internal polling in the DAGScheduler was polluting the logs.
(b) The Scala REPL logs were really noisy.
Merge pull request #384 from pwendell/debug-logs

Make DEBUG-level logs consummable.

Removes two things that caused issues with the debug logs:

(a) Internal polling in the DAGScheduler was polluting the logs.
(b) The Scala REPL logs were really noisy.