[WIP] [SPARK-2377] Python API for Streaming

This patch bring Python API for Streaming, WIP.

TODO:
updateStateByKey()
windowXXX()

This patch is based on work from @giwa
Take a look at https://github.com/apache/spark/pull/1628#issuecomment-50663192 for a discussion of a similar issue. It's possible to name this time.py without any conflicts; look at libraries like numpy, where you have numpy.random, to see that this is possible.