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.

Author: Fabian Hueske <fhueske@apache.org>
Author: Artem Tsikiridis <artem.tsikiridis@cern.ch>
Author: twalthr <info@twalthr.com>

Closes #143 from fhueske/hadoopFunctions and squashes the following commits:

f1b3f21 [Fabian Hueske] [FLINK-1076] Extended Hadoop Compatibility documentation to cover Hadoop functions
f4be4a0 [twalthr] [FLINK-1107] Hadoop Compatibility Layer documented
4a62bdb [Fabian Hueske] [FLINK-1076] Return type determined via type extraction. Added test cases. Minor improvements and clean-ups.
df947d9 [Artem Tsikiridis] [FLINK-1076] Extend Hadoop compatibility. Added wrappers for stand-alone Map, Reduce, and CombinableReduce functions.