Added wrappers for Hadoop functions

Tests and documentation included. Also tested on cluster.

I hijacked @twalthr's PR #131 to build the documentation of the Hadoop function wrappers on top.
That test checks if the combiner is correctly executed and will fail if not.
The collection-based execution does not run a combiner, because it does only make sense in a distributed setting to reduce the amount of shipped data.
Hence, the test fails if run on collections.