Address pull request comments

* Use crypto.Sign instead of directly calling secp256k1 lib
* Rename UserAccount to Account and Addr to Address (for consistency)
* Change AccountManager.Sign to take ptr to Account instead of
  address byte array
* Simplify copying of Accounts in Accounts()
* PubkeyToAddress and GetEntropyCSPRNG now exported

crypto/key.go

@@ -102,7 +102,7 @@ func NewKey(rand io.Reader) *Key {
 	id := uuid.NewRandom()
 	key := &Key{
 		Id:         id,
-		Address:    pubkeyToAddress(privateKeyECDSA.PublicKey),
+		Address:    PubkeyToAddress(privateKeyECDSA.PublicKey),
 		PrivateKey: privateKeyECDSA,
 	}
 	return key