Sub-hourly cycling intervals

[cylc]

This continues on from Issue #2. The internal cycling mechanism has been generalized, and we now have stepped yearly, monthly, and daily cycling, in addition to the original hours-of-the-day cycling.

[cylc]

The cycle time arithmetic module now handles minutes and seconds, and the generalized cycling mechanism should allow a cycler module that generates sequences with sub-hourly intervals. We just need to go through the code and check that all user inputs and internal use of cycle times can handle seconds (e.g. we may still assume or check for 10-digit cycle times of the form YYYYMMDDHH in places).

[hjoliver]

This will now be addressed in #119.