Initial WIP port to SparkR.

- Also add prettify's r and rd js files.
Updates. Add dataset download instructions.
Fixes and updates. Removed most Scala and Python snippets.
Remove codetabs div, tune code snippet layout.
More fixes and edits.
Turn homepage to add SparkR reference.
Remove redundant path.
Merge pull request #161 from concretevitamin/ampcamp5-sparkr

Add SparkR exercise.
add Tachyon back to the exercise
tweak tachyon exercise
offheap example
Small edits to tachyon page
Merge pull request #162 from manugoyal/ampcamp5

Small edits to tachyon page
tweak tachyon tutorial
tweak tachyon tutorial
tweak tachyon
Add new SparkR exercises
Merge branch 'ampcamp5' of https://github.com/amplab/training into ampcamp5
Fixes / tweaks to SparkR. Add a modern Spark UI screenshot.