[SPARK-7965] [SPARK-7972] [SQL] Handle expressions containing multiple window...

[SPARK-7965] [SPARK-7972] [SQL] Handle expressions containing multiple window expressions and make parser match window frames in case insensitive way

JIRAs:
https://issues.apache.org/jira/browse/SPARK-7965
https://issues.apache.org/jira/browse/SPARK-7972

Author: Yin Huai <yhuai@databricks.com>

Closes #6524 from yhuai/7965-7972 and squashes the following commits:

c12c79c [Yin Huai] Add doc for returned value.
de64328 [Yin Huai] Address rxin's comments.
fc9b1ad [Yin Huai] wip
2996da4 [Yin Huai] scala style
20b65b7 [Yin Huai] Handle expressions containing multiple window expressions.
9568b21 [Yin Huai] case insensitive matches
41f633d [Yin Huai] Failed test case.