AssetDir is now able to list the data root.

Previously, it was impossible to get a listing of entries right underneath
the data root.

Such a query is now possible by passing an empty string as data root.

Please be warned that I could only verify it to be working with my particular case, and therefore can't be sure I didn't break other cases. 
Maybe I missed tests to run, but if so, they are not runnable by `go test`.