Resolves https://github.com/mewwts/addict/issues/17
Implements prune_empty_list and prune_zero options for Dict::prune(...)
tagging no coverage for _ipython_display_()
Merge branch 'fix-dir-issue' - should make pulling the prune() changes easier
Merge pull request #22 from sabhiram/master

Implement prune_zero and prune_empty_list options for prune(...) - Resolves mewwts#17