Feature/file ds

- adds a File dataset interface, also Input/OutputFormatProvider 
- an implementation named FileDataset, including a definition and admin
- Map/reduce runner to support Input/OutputFormatProvider in addition to BatchReadable/Writable
- utilities for scoped runtime arguments ("dataset.<name>.my.property"). 
- unit tests

Please give feedback especially about the APIs. 
It seems a bit funny that it `split` here, but then concat back with `,` one by one. Should it be just `arguments.set(INPUT_PATHS, paths);`?