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.
In general, exceptions should be used for error handling, not for application logic (in my understanding).
Have a look at the WordCount example: https://github.com/apache/incubator-flink/blob/master/stratosphere-examples/stratosphere-java-examples/src/main/java/eu/stratosphere/example/java/wordcount/WordCount.java
it solves this issue differently.