Common utilities and programtic join.

[CarterFendley]

This PR contains the changes made first to dask-groupby for dask-join. Notable features:

• Extracts a common QueryRunner shared between both groupby and join.
  • Usage shown here
• Moves the question variables into a class attribute to maintain better encapsulation (example here).
• Change file names to standard python conventions
  • Renames groupby-dask.py -> groupby_dask.py
  • Renames join-dask.py -> join_dask.py
• Addition of a default "local" value for MACHINE_TYPE see here.

[Tmonster]

LGTM. Thanks!