File dialog can return badly-formed paths
Using the file dialog in ways such as this:
`[doc askForUserInput:"Export to…" ofType:2 initialValue:file_path]` 
can return paths that start with `file://`, which is not acceptable for the `com.bomberstudios.export_all_*` functions.  

This was observed on OSX 10.9.4, Sketch Version 3.0.4 (8053) and specifically for folders on the Desktop.

This should probably handled in a standard way.
