Use thiserror instead of derive_more (#44)

- Use thiserror instead of derive_more
- Format code
- Fix clippy warnings
- Add LICENSE_TEMPLATE

Signed-off-by: koushiro <koushiro.cqx@gmail.com>

src/codec.rs

@@ -1,3 +1,19 @@
+// Copyright 2019 Parity Technologies (UK) Ltd.
+// This file is part of substrate-subxt.
+//
+// subxt is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// subxt is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with substrate-subxt.  If not, see <http://www.gnu.org/licenses/>.
+
 use parity_scale_codec::Encode;
 
 #[derive(Clone)]