SPARK-884: Add unit test to validate Spark JSON output

This unit test simply validates that the outputs of
the JsonProtocol methods are syntactically valid JSON.
Updating assembly README to reflect recent changes in the build.
Merge pull request #894 from c0s/master

Updating assembly README to reflect recent changes in the build.
Address Patrick's comments
Fix line over 100 chars
Merge pull request #893 from ilikerps/master

SPARK-884: Add unit test to validate Spark JSON output
Merge remote-tracking branch 'upstreammesos/master' into sparkUISecurity