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

accounts/accounts_test.go

@@ -10,9 +10,8 @@ func TestAccountManager(t *testing.T) {
 	am := NewAccountManager(ks)
 	pass := "" // not used but required by API
 	a1, err := am.NewAccount(pass)
-	toSign := make([]byte, 4, 4)
-	toSign = []byte{0, 1, 2, 3}
-	_, err = am.Sign(a1.Addr, pass, toSign)
+	toSign := crypto.GetEntropyCSPRNG(32)
+	_, err = am.Sign(a1, pass, toSign)
 	if err != nil {
 		t.Fatal(err)
 	}