ISSUE #827 fix FLINK-827

Adding WordCountPLOJO example. It demonstrates how to use KeySelectors
in a word count example.
Removing comments and adding file support so that this example is
executable without additional data. While providing the functionality of
adding input and output files.
I think its nicer to do this "inline". Similar to the other wordcount example
```java
words.groupBy(new CustomizedWordKeySelector())
  .reduce(new CustomizedWordReducer());
```