Add scheduler mode to wikimetrics

In addition to the web, queue, and import modes, wikimetrics can now run
in scheduler mode.  This reads schedules from wikimetrics/scheduler and
runs tasks in the wikimetrics/schedules/[time period].py files.  When
this is run in development mode, daily tasks will run much more often so
you can test without waiting a day.  An example no-op daily task is
included with this change to show how everything ties together.

Card: analytics 1378
Change-Id: I1ecf45cf9f846a9058e7e84ee1bc376f1b1037b7