Update the "Running Spark on Tachyon" docs

Add a section to "Running Spark on Tachyon", which is about how to
persist Spark RDDs into Tachyon.
Merge pull request #270 from PasaLab/docs-Spark-OFF_HEAP

Update the user documentation "Running Spark on Tachyon"
improve Persist Spark RDDs into Tachyon doc
prepare to release 0.5.0
revert back to the last commit. do the release work in the release branch