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. 
Shall we pass the `classLoader` as well? If the `InputFormat` / `OutputFormat` class is actually in user program jar, you can only load it through the `classLoader` provided here, as `Class.forName()` will try to look it up from the CDAP system `ClassLoader` (that's what the ClassLoader of `FileDataset`), which will fail.