[sql] Rename execution/aggregates.scala Aggregate.scala, and added a bunch of...

[sql] Rename execution/aggregates.scala Aggregate.scala, and added a bunch of private[this] to variables.

Author: Reynold Xin <rxin@apache.org>

Closes #348 from rxin/aggregate and squashes the following commits:

f4bc36f [Reynold Xin] Rename execution/aggregates.scala Aggregate.scala, and added a bunch of private[this] to variables.