Added i?zip_dicts()
Tests for zip_values() and zip_dicts()
Better error when i?zip_values() passed not dicts
Docs for i?zip_dicts() and example for izip_values()
Docs for last() and butlast()
Tests for last() and butlast()
Updated with_prev() docs

- added fill argument,
- added ipartition_by to see also.
Removed underscore from is_none() and not_none()
Updated TODO list and sorted it by priority