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
Review comments
Updated cluster diagram to show caches
Fix unit test failure due to changed default
Fix some review comments
Merge pull request #905 from mateiz/docs2

Job scheduling and cluster mode docs