Add scheduling support for layouts

[plural]

We should be able to specify a start and end time for a layout.

Proposal:

• add start_time and end_time entries to layouts in the playlist section of config.json
  • should be in YYYY-MM-DD HH:MM format and is always assumed to be local time.
• validate that there are no overlaps with the layouts and reject the config if overlaps are present.
• layout state machine should be responsible for periodically checking the playlist for scheduled events (starts or stops). If nothing is scheduled, it will be business as usual.