Implement MtProto 2.0 (closes #484, thanks @delivrance!)

Huge shoutout to @delivrance's pyrogram, specially this commit:
pyrogram/pyrogram/commit/42f9a2d6994baaf9ecad590d1ff4d175a8c56454

telethon/network/mtproto_sender.py

@@ -156,17 +156,7 @@ class MtProtoSender:
 
         :param message: the TLMessage to be sent.
         """
-        plain_text = \
-            struct.pack('<qq', self.session.salt, self.session.id) \
-            + bytes(message)
-
-        msg_key = utils.calc_msg_key(plain_text)
-        key_id = struct.pack('<Q', self.session.auth_key.key_id)
-        key, iv = utils.calc_key(self.session.auth_key.key, msg_key, True)
-        cipher_text = AES.encrypt_ige(plain_text, key, iv)
-
-        result = key_id + msg_key + cipher_text
-        self.connection.send(result)
+        self.connection.send(utils.pack_message(self.session, message))
 
     def _decode_msg(self, body):
         """
@@ -175,34 +165,14 @@ class MtProtoSender:
         :param body: the body to be decoded.
         :return: a tuple of (decoded message, remote message id, remote seq).
         """
-        message = None
-        remote_msg_id = None
-        remote_sequence = None
+        if len(body) < 8:
+            if body == b'l\xfe\xff\xff':
+                raise BrokenAuthKeyError()
+            else:
+                raise BufferError("Can't decode packet ({})".format(body))
 
         with BinaryReader(body) as reader:
-            if len(body) < 8:
-                if body == b'l\xfe\xff\xff':
-                    raise BrokenAuthKeyError()
-                else:
-                    raise BufferError("Can't decode packet ({})".format(body))
-
-            # TODO Check for both auth key ID and msg_key correctness
-            reader.read_long()  # remote_auth_key_id
-            msg_key = reader.read(16)
-
-            key, iv = utils.calc_key(self.session.auth_key.key, msg_key, False)
-            plain_text = AES.decrypt_ige(
-                reader.read(len(body) - reader.tell_position()), key, iv)
-
-            with BinaryReader(plain_text) as plain_text_reader:
-                plain_text_reader.read_long()  # remote_salt
-                plain_text_reader.read_long()  # remote_session_id
-                remote_msg_id = plain_text_reader.read_long()
-                remote_sequence = plain_text_reader.read_int()
-                msg_len = plain_text_reader.read_int()
-                message = plain_text_reader.read(msg_len)
-
-        return message, remote_msg_id, remote_sequence
+            return utils.unpack_message(self.session, reader)
 
     def _process_msg(self, msg_id, sequence, reader, state):
         """