Attempt to extract priorities from redmine responses appropriately.

This should hopefully fix #154.

This makes the redmine service override the base service's ``get_priority``
method to extract the priority correctly from the response, and also adds a
``PRIORITY_MAP`` to map the default ids onto taskwarrior priorities.

Untested!  I don't have a redmine instance still to mess with, so any critical
feedback is much appreciated.