Add accounts package and refactor key stores

* Add initial UserAccount and AccountManager structs
* Add NewAccount, Sign and Accounts functions
* Refactor key stores to use key address as main identifier
  while keeping the UUID.
* Use key address as file/dir names instead of UUID

crypto/crypto.go

@@ -134,7 +134,7 @@ func ImportPreSaleKey(keyStore KeyStore2, keyJSON []byte, password string) (*Key
 		return nil, err
 	}
 	id := uuid.NewRandom()
-	key.Id = &id
+	key.Id = id
 	err = keyStore.StoreKey(key, password)
 	return key, err
 }
@@ -167,9 +167,10 @@ func decryptPreSaleKey(fileContent []byte, password string) (key *Key, err error
 	ecKey := ToECDSA(ethPriv)
 	key = &Key{
 		Id:         nil,
+		Address:    pubkeyToAddress(ecKey.PublicKey),
 		PrivateKey: ecKey,
 	}
-	derivedAddr := ethutil.Bytes2Hex(key.Address())
+	derivedAddr := ethutil.Bytes2Hex(key.Address)
 	expectedAddr := preSaleKeyStruct.EthAddr
 	if derivedAddr != expectedAddr {
 		err = errors.New("decrypted addr not equal to expected addr")
@@ -223,3 +224,8 @@ func PKCS7Unpad(in []byte) []byte {
 	}
 	return in[:len(in)-int(padding)]
 }
+
+func pubkeyToAddress(p ecdsa.PublicKey) []byte {
+	pubBytes := FromECDSAPub(&p)
+	return Sha3(pubBytes[1:])[12:]
+}