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. 
Please look at https://github.com/caskdata/cdap/blob/develop/pom.xml for how to do the copyright block. If you copied it from somewhere else, that place probably needs correcting.