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). 
Let's derive this from `object` for now, and add a parent class for all declarative service bindings (perhaps `Declaration`? @nedbat, in your role as chief sloganeer: go!)

That way, you won't run afoul of existing edx-platform code that iterates over `.fields` and calls `is_set_on` and the like.