hadoopcompatibility: Implementations of basic programming interfaces and...

... a basic driver.

* wrappers for Mapper, Reducer and Combiner (as a local Reducer) on the new Java API
* wrapper for OutputCollector
* wrappers for Partitioner, values comparator.
* a driver making it posible to run unmonitored (so far)  Hadoop jobs on Flink.
* tests and variations of the WordCount driver exclusively in Hadoop.

You can find more about the Google Summer of Code project here:  https://issues.apache.org/jira/browse/FLINK-838?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14031511
The constructor initializes `this.combiner` to the combine class registered in the `JobConf`. If the combiner is the same class as the reducer it is still registered in the `JobConf`, right?

In that case, the combine method may not be called if no combiner is registered, i.e.,  `this.combiner == null`. Instead, this is a invalid system behavior and an exception should be raised.