Add possibility to choose celery as task backend

Never worked a lot with pull requests and didn't now that all new commits will be added to it automatically. So I closed previous one and made new one.

I've fixed one issue with imports which caused that it was impossible to install using setup.py and added possibility to use celery as task backend instead of kronos. More info in updated README.
With celery it is possible to ratelimit tasks or make some locking. If so, we could get rid of .is_measuring flag which causes that skwissh sometimes stops to collect data for some servers.