Merge pull request #257 from andyk/add-version-vars-to-docs

docs/README.md

docs/_config.yml

docs/_layouts/global.html

docs/api.md

docs/bagel-programming-guide.md

docs/configuration.md

docs/contributing-to-spark.md

docs/ec2-scripts.md

docs/index.md

docs/java-programming-guide.md

docs/quick-start.md

docs/running-on-mesos.md

docs/scala-programming-guide.md

docs/spark-standalone.md

docs/tuning.md