Add Weekly Summary Report

- introduce PrevNextWeekMixin
- add WeeklySummaryReportCalculator
- add interval_to_hours template tag
skip postgresql specific test when using sqlite

I'd rather not have jenkins running tests against postgresql quite yet,
so I set it up to skip the PG specific tests when not using that engine.

TravisCI now explicitly runs 'make travis', which runs the tests
using using settings_integration, which points at postgres.

So, between travis and jenkins, we ought to be good. And this should let
us avoid paying the costs of setting up postgres on the jenkins server
and slowing down all the jenkins tests by running them against PG. Especially
since in the long run, we want to eliminate the PG specific queries anyway.
Merge branch 'nikolas-weekly-summary-report'