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. 
Looks like we want the following:
* Write to a named file dataset unique within a namespaced application (app://somefile, somefile -> /cdap/namespaces/somenamespace/someapp/data)
* Write to a named file dataset unique within a namespace (ns://somefile -> /cdap/data/somenamespace/data)
* Write to a named file dataset anywhere in the filesystem (/sdf/zcxv/somefile)