First pass at presenter cleanup.
US7 Simplify presenters.

Remove the idea of the NullPresenter because it's not the one that
should be null-objected. The presentee is the thing that can be null, so
null-objectify that thing.
Rename Statistics to Dashboard.

I'm thinking that the home page of the site is a dashboard.
That's the best name I have right now.
Slight simplification of fetching best/worst team from DashboardReport.
DRY up the each_{presentee} methods between presenters.
Merge pull request #5 from winterchord/cleanup_presenters

Fix Card(1).