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 have not looked at all of the file systems support, but the S3FS, which is most likely what we'd use in prod, is completely lazy. __init__ does nothing except store parameters in class-local variables. Calling things like open() in turn call (ultimately) _s3conn(), which will establish the connection if it either doesn't exist or is stale. 

So there should be no performance issue.