[SPARK-2218] rename Equals to EqualTo in Spark SQL expressions.

Due to the existence of scala.Equals, it is very error prone to name the expression Equals, especially because we use a lot of partial functions and pattern matching in the optimizer.

Note that this sits on top of #1144.

Author: Reynold Xin <rxin@apache.org>

Closes #1146 from rxin/equals and squashes the following commits:

f8583fd [Reynold Xin] Merge branch 'master' of github.com:apache/spark into equals
326b388 [Reynold Xin] Merge branch 'master' of github.com:apache/spark into equals
bd19807 [Reynold Xin] Rename EqualsTo to EqualTo.
81148d1 [Reynold Xin] [SPARK-2218] rename Equals to EqualsTo in Spark SQL expressions.
c4e543d [Reynold Xin] [SPARK-2210] boolean cast on boolean value should be removed.