Makes log record propagation a bit saner

I don't understand how this is not a Python bug, but the users seem
happy about this. It is terrible.
Makes LevelCheckerLogger instanciation safer
Prevents "no handlers for logger "root"" warning
Renames debug's logger VisLog -> vistrails
Different approach -- two loggers

The logger that the 'debug' module logs on is now vistrails.logger. It
is internal and API users should NOT configure it (change level or
attach filters/handlers to it). It is controlled by the 'debug' module.

This internal logger re-logs to a 'vistrails' logger, which is meant to
be configured by API users if needed -- changing the level of this one
won't break anything.
Importing debug no longer adds a console handler

This means that a root logger should either be configured (usually the
case when VisTrails is used by something else) or log_to_console()
should be called to add the console StreamHandler to the root logger.
Makes set_message_level() change 'vistrails' level
Merge branch 'logger-propagation' into 'master'