Plot is now a subclass of MapFeature

In preparation for supporting other types of map features:
* Update tiler SQL to be MapFeature-centric rather than Plot-centric
* Update "plot" to "mapFeature" in variable/function names

But don't change the app/tiler interface yet, to keep things simpler until we actually support new feature types.
Merge pull request #29 from RickMohr/topic/plot-is-a-map-feature

Plot is now a subclass of MapFeature