SPARK-1173. Improve scala streaming docs.

Clarify imports to add implicit conversions to DStream and
fix other small typos in the streaming intro documentation.

Tested by inspecting output via a local jekyll server, c&p'ing the scala commands into a spark terminal.

Author: Aaron Kimball <aaron@magnify.io>

Closes #64 from kimballa/spark-1173-streaming-docs and squashes the following commits:

6fbff0e [Aaron Kimball] SPARK-1173. Improve scala streaming docs.