Uses low 's' values for signatures
Moves 'low S value' test from eckey.js to ecdsa.js
Merge pull request #135 from greenaddress/low-s-signatures

Uses low 's' values for signatures
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