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 would be good to be able to do that. But how? We can only do that if we do not allow to specify a base path, and always generate it from the dataset name (and namespace). But that would make it difficult to use files that are already on HDFS. I am not sure what is right here.