SPARK-1121: Include avro for yarn-alpha builds

This lets us explicitly include Avro based on a profile for 0.23.X
builds. It makes me sad how convoluted it is to express this logic
in Maven. @tgraves and @sryza curious if this works for you.

I'm also considering just reverting to how it was before. The only
real problem was that Spark advertised a dependency on Avro
even though it only really depends transitively on Avro through
other deps.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #49 from pwendell/avro-build-fix and squashes the following commits:

8d6ee92 [Patrick Wendell] SPARK-1121: Add avro to yarn-alpha profile
Add Jekyll tag to isolate "production-only" doc components.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #56 from pwendell/jekyll-prod and squashes the following commits:

1bdc3a8 [Patrick Wendell] Add Jekyll tag to isolate "production-only" doc components.
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.
SPARK-1173. (#2) Fix typo in Java streaming example.

Companion commit to pull request #64, fix the typo on the Java side of the docs.

Author: Aaron Kimball <aaron@magnify.io>

Closes #65 from kimballa/spark-1173-java-doc-update and squashes the following commits:

8ce11d3 [Aaron Kimball] SPARK-1173. (#2) Fix typo in Java streaming example.