Migrates ECKey to stricter API
Updates README.md to stricter ECKey API
Cleans up ECKey error handling
Adds assertion error messages for HDWallet
Prefer Array.prototype.slice over map
Merge pull request #141 from dcousens/slicenotmap

Prefer Array.prototype.slice over map
Merge pull request #139 from dcousens/eckeystrict

Migrates ECKey to stricter API
Merge pull request #142 from dcousens/asserts

Adds assertion error messages for HDWallet