Various fixes, often in SQL

This builds off of https://github.com/ContinuumIO/blaze/pull/270 and https://github.com/ContinuumIO/blaze/pull/272

No theme here other than that I'm building a demo and fixing everything that breaks.  Mostly this includes sqlalchemy stuff.

* By operation includes grouping columns in schema.
* SQL Groupby now includes the grouping columns
* Reductions like `t['amount'].sum()` now have schema `{amount: real}` rather than `real`.  Thoughts on this @mwiebe  ?
* Use sqlalchemy.label to maintain column names
* Python now includes streaming by operation in simple/common case.  This is a kludge.