cc rxin, please take a quick look, I'm working on tests.

Author: Davies Liu <davies@databricks.com>

Closes #6238 from davies/readwrite and squashes the following commits:

c7200eb [Davies Liu] update tests
9cbf01b [Davies Liu] Merge branch 'master' of github.com:apache/spark into readwrite
f0c5a04 [Davies Liu] use sqlContext.read.load
5f68bc8 [Davies Liu] update tests
6437e9a [Davies Liu] Merge branch 'master' of github.com:apache/spark into readwrite
bcc6668 [Davies Liu] add reader amd writer API in Python