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). 
I don't know what pyfilesystem is doing under the covers, but I'd like to understand a little better what the implications are for processes like grading that instantiate the full course tree.

1. Do we take the performance hit for file system access when the XBlock is instantiated? Or is it delayed until you try to use it for something?
2. Do we have to worry about open file descriptors when we’re doing long running processes like grading? We’ve had problems in the past where a sandbox-whitelisted course couldn’t be graded because the way one of its problems was coded was leaving open file handles and ran into system limits.