Release v0.0.20

Changelog

fa889df Add basic helper functions for hd_key.go
f52f8ff Added nil checks to reduce potential panics
6cc0f58 Added test coverage & examples for new methods
d731652 Enable shadow
4b5120f Merge pull request #3 from galt-tr/hdkeyHelpers
5817c46 Moved todo into a Github issue for better tracking
ad62ee7 Updated makefile
d562c4d Upgraded deps