Use peewee ORM instead of raw sql.

This ended up changing or restructuring basically everything.

* No more McBenchClient, which wrapped a sqlite connection and did all
  the queries; instead db interactions are done through the model
  classes in mcbench.models.

* Query logic (bits from from mcbench.benchmark, bits from
  mcbench.query) has been mostly centralized in a new mcbench.querier
  module. (This should have been done before, but became more
  obvious when Benchmark became a model class.)

* New drop_tables and create_tables management commands.

* I now lets things refer to settings directly, instead of having to
  go through app.config.

* Tests pass, but I haven't gone through and looked at which still
  make sense, and what more should be added.