Add Weekly Summary Report

- introduce PrevNextWeekMixin
- add WeeklySummaryReportCalculator
- add interval_to_hours template tag
manage command for moving attachments to S3

This is a one-time operation, which is why some stuff is hard-coded and doesn't come
with tests.

All it's doing is going through every attachment in the database that the PMT has
stored locally and shipping it up to the configured S3 bucket, using the same
path/filename scheme that we've established elsewhere. Then it updates the db
entry with the new S3 URL.

Once this goes out to production, I will rsync the uploaded attachments from hutz
(where the PMT lives) to the /mnt/nfs/applications/dmt/attachments/ directory,
then run './manage.py move_attachments_to_s3 --settings...' and that should
be that.

Once everything has been moved up and verified, I'll delete this command
since there will no longer be any use for it.
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'
Merge pull request #83 from thraxil/attachments-to-s3

manage command for moving attachments to S3
move to s3 command has been run, so it's no longer needed
Merge pull request #85 from thraxil/remove-move-to-s3-command

move to s3 command has been run, so it's no longer needed