Add javaDoc comments to ExecutionEnvironment (final), DataSet (parts), InputTypeConfigurable, and ResultTypeQueryable.

  - some KMeans Variants
  - some WordCount variants

Add utility test to check string and tuple sorting (not active by default, time consuming and Linux OS dependent)

Rename UnilateralSortmerger -> ExternalSorter (part 1)
Fix checkstyle warning.

Add a compare method to teh comparators that works directly on objects.

uses values of passed keys
can skip parts of the tuple if necessary

disable the MatchJoinCancelingITCase test to speed up travis builds