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). 
Once you aren't derived from `Field`, then you should follow https://docs.python.org/2/howto/descriptor.html#descriptor-protocol to make a read-only data descriptor.