Tasks refactorization

Separates tasks from the way they can be actually
executed
New management commands

Allows installing tasks using different backends. Possible
backends at the moment:
- kronos
- celery

Methods added:
- installsensors
- uninstallsensors

Backend is determined by the variable
SKWISSH_TASK_BACKEND in settings.py. Default value
 is 'kronos'.
Information in README about how to change task backend
installtasks changed to installsensors in README

Changed because new method can be used for both
backends and user should now choose this one. If not
 so he can make misteke if switches to celery.