1. added explain()
2. add isLocal()
3. do not call show() in __repl__
4. add foreach() and foreachPartition()
5. add distinct()
6. fix functions.col()/column()/lit()
7. fix unit tests in sql/functions.py
8. fix unicode in showString()

Author: Davies Liu <davies@databricks.com>

Closes #4645 from davies/df6 and squashes the following commits:

6b46a2c [Davies Liu] fix DataFrame Python API

(cherry picked from commit d8adefef)
Signed-off-by: Michael Armbrust <michael@databricks.com>