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). 
Based on our conversation about restructuring field, can you just override any methods that don't make sense for this block to return either something sensible, or raise useful errors?