Adds a file system field

This adds a file system field type. Relies on a file system service, so this is useless outside of the context of a runtime. There is a matching branch in xblock-sdk which adds this service, and includes tests (which also test my last patch to XBlock). 
The choice of 'fs' was for consistency with existing naming. Specifically, this provides access to a version of: 

  https://code.google.com/p/pyfilesystem/
 
They've opted for this naming conventions: 

  pip install fs
  import fs
  etc. 

As opposed to pyfilesystem, pyfs, fs, filesystem, etc.