Add support for optional debug parameters
Update debug documentation for clarity and accuracy
Add printFields and printTuple interval support
sumByLocalKeys, Counters and Task Numbers
Merge branch 'release/0.9.0rc4' into develop
Minor fixes.
Replaced get with getOrElse in FileSource createTap.
Merge pull request #676 from stephanh/minor_fixes

Minor fixes.
Fixed bug in package.scala.
Bumped version to 0.9.0rc4 in package.scala.
Throw a more informative error when failing to validate sources.
Merge pull request #683 from azymnis/validation_error

Throw a more informative error when failing to validate sources.
Merge pull request #682 from stephanh/bug_fix

Fixed bug introduced in #676
Added failing test for matrix2 serialization.
Changed chill version to 0.3.5.
Fixed serialisation issues in Matrix2 Product.
Changed Matrix2 tests to use runHadoop instead of run.
Merge pull request #684 from stephanh/matrix2_serialization

Matrix2 serialisation has issues
Added debug method to ValuePipe.
Clean up Args+Mode coupling