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). 
Discussed in person. Summary: 
* Expense depends on type of filesystem. It's cheap on devstack, but it won't necessarily be cheap in prod. 
* User of the filesystem has a better idea of the access patterns, and so knows whether to cache. 
* Fields have a pre-built caching mechanism. One caching mechanism is better than two.