Make it easier to add new plots/tables

[dhmay]

Right now, adding a new plot or table isn't very easy or intuitive for a coder who isn't me.

I should add JamstatsPlot and JamstatsTable base classes, make all the existing plots and tables into subclasses of those base classes, and make it easy to add new plots/tables and control what gets displayed.

That last part should apply both to the web interface and to the pdf generation.