ENH: added offset_spines function to utils and trim kwarg to despine
DOC: demo offset_spines and despine(trim=True) in an existing examples

Run hexstrip on aesthetics notebook
Tweak docstrings for new spine functions

Update docs with info on offset_spines
Add unit tests for spine utilities
Merge pull request #124 from mwaskom/phobson-offset-spines

ENH: function to offset spines
Add default value for offset_spines
Merge pull request #125 from mwaskom/default_offset

Add default value for offset_spines
Remove vestigal `violin` function
Fix merge conflict with spine tests