Remove all empty directories from index.

This removes empty dirs as much as possible, so fixes #79

- sneaky_index helps preventing empty forbidden namespaces
(especially when nodes are not unfolded/not loaded for some levels), but
can hide deeper allowed nodes.
- this removing of empty dirs is only performed for the sort options:
tsort, dsort,msort,nsort and hsort.
Remove fallback for config setting 'sneaky_index'

Fixes #101
Merge pull request #102 from samuelet/emptydirs

Remove all empty directories from index.