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.