Split MapOutputTracker into Master/Worker classes.

Previously, MapOutputTracker contained fields and methods that
were only applicable to the master or worker instances.  This
commit introduces a MasterMapOutputTracker class to prevent
the master-specific methods from being accessed on workers.

I also renamed a few methods and made others protected/private.
Fix test failures in local mode due to updateEpoch
Unwrap a long line that actually fits.
Docs: Fix links to RDD API documentation
Merge pull request #82 from JoshRosen/map-output-tracker-refactoring

Split MapOutputTracker into Master/Worker classes

Previously, MapOutputTracker contained fields and methods that were only applicable to the master or worker instances.  This commit introduces a MasterMapOutputTracker class to prevent the master-specific methods from being accessed on workers.

I also renamed a few methods and made others protected/private.
Merge pull request #98 from aarondav/docs

Docs: Fix links to RDD API documentation