Removing some intermediate values

This removes some intermediate steps that were ultimately not necessary
to the math.  It also consolidates some of the methods for counting events
so that fewer intermediate values are necessary.

To that end, a new scope is added to the ToDo model for events that are
*either* created_at or completed_at after a certain date. This scope
allows the StatsController to pull out the largest possible responsive
set of values, and then filter just the particular slices that it needs
for various steps in the calculation.
Removing some more intermediate values

Also removing the python-esque combination methods to set multiple
disparate variables from a single method by refactoring that to a
simpler method.

Reducing the complexity of computing running averages.

Note: It is not clear to me why the actions_done_lastyears_data
action is entirely unscoped on date. That seems to be the expected
behavior, so I left it alone.
Combining the running average methods
Removing a single-line method
Tweak to running averages
Simplufying interpolation method
Moving time labels to separate methods
Moving label creation to helpers
Merge pull request #195 from doncruse/master

Removing some intermediate values and shifting HTML formatting to a helper