Support for creating and mounting datasets from a local path

[epwalsh]

Related to #13. See #13 (comment).

This is a little more complex because we probably don't want to re-upload those local files each time gantry run is called if they haven't changed. Maybe we add a flag to force re-upload, otherwise we could compute a hash of the contents of all source files and only upload when the hash changes. But then we're back to re-implementing git...