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 can be simplified as

`String line = CharStreams.readFirstLine(CharStreams.newReaderSupplier(Locations.newInputSupplier(location), Charsets.UTF_8));`

which handle open/read/close of the stream.