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. 
Missed the trim. Since we are using regex (`split` takes regex), we can use regex to trim. Or, better yet, use Guava `Splitter` to help `Splitter.on(',').omitEmptyStrings().trimResults().split(pathsArg)`