Improving the graphx-programming-guide.
Merge pull request #423 from jegonzal/GraphXProgrammingGuide

Improving the graphx-programming-guide

This PR will track a few minor improvements to the content and formatting of the graphx-programming-guide.
Additional edits for clarity in the graphx programming guide.
Broadcast variable visibility change & doc update.

Note that previously Broadcast class was accidentally marked as private[spark]. It needs to be public
for broadcast variables to work. Also exposing the broadcast varaible id.
Updated API doc for Accumulable and Accumulator.
Added package doc for the Java API.
Added license header for package.scala in the Java API package.
Maintain Serializable API compatibility by reverting back to java.io.Serializable for Broadcast and Accumulator.
Fixed a typo in JavaSparkContext's API doc.
Don't clone records for text files
Deprecate rather than remove old combineValuesByKey function
Merge pull request #425 from rxin/scaladoc

API doc update & make Broadcast public

In #413 Broadcast was mistakenly made private[spark]. I changed it to public again. Also exposing id in public given the R frontend requires that.

Copied some of the documentation from the programming guide to API Doc for Broadcast and Accumulator.

This should be cherry picked into branch-0.9 as well for 0.9.0 release.
Adding fix covering combineCombinersByKey as well
Style fix
Add GraphX dependency to examples/pom.xml
Merge pull request #427 from pwendell/deprecate-aggregator

Deprecate rather than remove old combineValuesByKey function
Merge pull request #429 from ankurdave/graphx-examples-pom.xml

Add GraphX dependency to examples/pom.xml
Merge pull request #428 from pwendell/writeable-objects

Don't clone records for text files
Describe GraphX caching and uncaching in guide
Merge pull request #431 from ankurdave/graphx-caching-doc

Describe caching and uncaching in GraphX programming guide