Add more tests and docs for DataFrame Python API, improve test coverage, fix bugs.

Author: Davies Liu <davies@databricks.com>

Closes #4331 from davies/fix_df and squashes the following commits:

dd9919f [Davies Liu] fix tests
467332c [Davies Liu] support string in cast()
83c92fe [Davies Liu] address comments
c052f6f [Davies Liu] Merge branch 'master' of github.com:apache/spark into fix_df
8dd19a9 [Davies Liu] fix tests in python 2.6
35ccb9f [Davies Liu] fix build
78ebcfa [Davies Liu] add sql_test.py in run_tests
9ab78b4 [Davies Liu] Merge branch 'master' of github.com:apache/spark into fix_df
6040ba7 [Davies Liu] fix docs
3ab2661 [Davies Liu] add more tests for DataFrame

(cherry picked from commit 068c0e2e)
Signed-off-by: Reynold Xin <rxin@databricks.com>