Additional edits for clarity in the graphx programming guide.
Describe GraphX caching and uncaching in guide
Merge pull request #431 from ankurdave/graphx-caching-doc

Describe caching and uncaching in GraphX programming guide
Merge pull request #424 from jegonzal/GraphXProgrammingGuide

Additional edits for clarity in the graphx programming guide.

Added an overview of the Graph and GraphOps functions and fixed numerous typos.