Add KijiExpress music tutorial development section.
Added apidocs page link to KijiExpress ScalaDocs.

Reviewed by: renuka
Added accordian boxes.

Reviewed by: renuka
Update tutorial code and explanations to reflect changes.

Reviewed by: juliet
CHOP-70. Update the documents to reflect the removal of the EXPRESS_CLASSPATH.

Fixed a few typos and changed the scripts to use the --libjars command line arguments.
Explain sortNextSongs in topnextsongs of Express Music tutorial.

Reviewed by: juliet
Fixed error in name of kiji-express artifact.

Reviewed by: renuka
Changed overview to discuss the Scalding tuple model.

Reviewed by: renuka
Recommendation generator.

Reviewed by: kiyan
Fixup KijiExpress API docs links.

Reviewed by: juliet