Import calendar

@anthonywebb, this is a quick addition for accessing Weather Underground to adjust the zones watering duration.

The adjustment formula was stolen from OpenSprinkler: this means that this should be under the GPL.

There is also a minor change to calendar: beside a better module documentation, the code now proceeds with periodic updates (one every 12 hours), as inspired by the method used for weather updates.

Testing has been light: I did unit test the calendar and weather modules, but this is about it.