Provide docs to describe running on CDH/HDP cluster.

This doc consolidates information relevant to CDH/HDP users in a single place.
Changes based on feedback
File rename
Adding Apache license to two files
Merge pull request #904 from pwendell/master

Adding Apache license to two files
Merge pull request #900 from pwendell/cdh-docs

Provide docs to describe running on CDH/HDP cluster.
Work in progress:

- Add job scheduling docs
- Rename some fair scheduler properties
- Organize intro page better
- Link to Apache wiki for "contributing to Spark"
More fair scheduler docs and property names.

Also changed uses of "job" terminology to "application" when they
referred to an entire Spark program, to avoid confusion.
Added cluster overview doc, made logo higher-resolution, and added more
details on monitoring
Some tweaks to CDH/HDP doc