Add a logger for dbmigrator that writes to stdout

[karenc]

For example in a migration file 20160128111115_mimetype_removal_from_module_files.py:

from dbmigrator import logger

logger.info('message from migration')

You will see this when you run the migration:

[INFO] dbmigrator (20160128111115_mimetype_removal_from_module_files.py) - message from migration

Close #3