Spark compute

fixes to Selection, Projection, By. Added ReLabel. needed for Jaccard calculation.
Also some PEP8 fixes.
Right, so `t['x'] > (t['y'] + 1)` would fail, correct?  I actually think that getting this to work with in Spark with the current `TableExpr` system is pretty difficult.  I'm working to improve with with Scalar Exprs.  I do think that we should be mindful of where our implementations aren't fully general though.  In particular I think we should document things like this, perhaps as comments with `TODO` markings.  Are there other solutions in the compute backend that aren't fully general?

What is here looks like a good improvement over what was there before.