Configurable list of patterns which will be ignored when pruning empty directories.

* util.prune_dirs modified to accept glob patterns as clutter to determine emptiness.
* config option, 'clutter' (a list of filenames/glob patterns)
* ImportTask.prune passes this option's value to prune_dirs.
Merge pull request #93 from steinitzu/prune-dirs-clutter

Customizeable clutter when pruning directories