benchmark for s2 covering

it's a little gross to run, but it works.
upgrade to sbt 0.12.1, alternate covering impl

- update to 0.12.1 so we can use a plugin for running benchmarks
- coverAtAllCells_LessNaive just gets the covering with the smallest
  cells (i.e. the max level) and then returns it and their parents.
fix all-level s2cover
benchmark for s2 covering

it's a little gross to run, but it works.
fix cover-all impls
delete benchmark code
final fixes to s2 cover all, omg
.8 bump