Added cache abstract class.

This lets groups and caches (now referred to as nodes) be treated in a uniform way.
Fixed groups command help text.
Added BanSubmission model fields and full BanSubmissionItem model
BanSubmission instances creation when submitting a ban
Split initialization process for further control.

Core initialization has been splitted into request intialization, management command
initialization, task initialization and worker initialization so different things can
be done in each place.
Added base_url config setting.

This will be useful for periodical tasks that may need access to this information
when not in a request context (e.g. to render an e-mail to be sent to an user).
Ban submission notifications
Simplified ban submissions notification e-mail
Some 0.2.1 pre-release adjustments
Merge branch 'cache-lists' into dev
Merge branch 'master' into dev
logging.handlers.NullHandler only supported in Python 2.7. Replaced by WatchedFileHandler using /dev/null in productions
Cleaned up unused imports
Merge branch 'dev' (cache-lists release)