Author: Davies Liu <davies@databricks.com>

Closes #4351 from davies/python_udf and squashes the following commits:

d250692 [Davies Liu] fix conflict
34234d4 [Davies Liu] Merge branch 'master' of github.com:apache/spark into python_udf
440f769 [Davies Liu] address comments
f0a3121 [Davies Liu] track life cycle of broadcast
f99b2e1 [Davies Liu] address comments
462b334 [Davies Liu] Merge branch 'master' of github.com:apache/spark into python_udf
7bccc3b [Davies Liu] python udf
58dee20 [Davies Liu] clean up