add an example that runs wordcount over file datasets

simple example to read and write files. 
- a service that can upload/download files
- a mapreduce that reads/writes the same files, with runtime arguments to specify the paths

I am open to renaming this, if you have a better suggestion. 
two ees looks odd. 
co.cask.cdap.examples.filedataset.FileDatasetExample  perhaps?