Proper open/close handling of combine stubs in combining sorter.
Combine stubs are lazily initialized upon spilling.
Adjusted combining sorter test cases.
Updated contributors.
Added cobertura Maven plugin for test coverage report generation
Added JDBC Input Formant and tests.
Fixed serialization warnings.
Marked transient fields.
Minor formattings.
Adjusted JDBC example to use internal derby DB for better debugging support.
Added DefaultInputSplitAssigner as default fallback for GenericInputSplit.
Fixed handling of unsplittable inputs and added sanity checks.
Removed redundant split generation in JDBC format after fixing unsplittabel input behavior.