Improve dynamic events type sizes API (#61)

* Do not error if type sizes missing: log warn

* Split out EventsSubscriber, allow updating events decoder

* Format code

* Check missing type sizes after registration

* Add example which for submit and watching events

examples/submit_and_watch.rs

@@ -0,0 +1,54 @@
+// 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 futures::future::Future;
+use substrate_subxt::{balances, system::System, DefaultNodeRuntime as Runtime};
+use sp_keyring::AccountKeyring;
+
+type AccountId = <Runtime as System>::AccountId;
+type Balance = <Runtime as balances::Balances>::Balance;
+
+fn main() {
+    env_logger::init();
+    let signer = AccountKeyring::Alice.pair();
+
+    let dest = AccountKeyring::Bob.to_account_id();
+
+    let fut = substrate_subxt::ClientBuilder::<Runtime>::new()
+        .build()
+        .and_then(|cli| cli.xt(signer, None))
+        .and_then(move |xt| {
+            xt.watch()
+                .events_decoder(|decoder| {
+                    // for any primitive event with no type size registered
+                    decoder.register_type_size::<(u64, u64)>("IdentificationTuple")
+                })
+                .submit(balances::transfer::<Runtime>(dest.clone().into(), 10_000))
+        });
+
+    let mut rt = tokio::runtime::Runtime::new().unwrap();
+    match rt.block_on(fut) {
+        Ok(extrinsic_success) => {
+            match extrinsic_success.find_event::<(AccountId, AccountId, Balance, Balance)>("Balances", "Transfer") {
+                Some(Ok((_from, _to, value, _fees))) =>
+                    println!("Balance transfer success: value: {:?}", value),
+                Some(Err(err)) => println!("Failed to decode code hash: {}", err),
+                None => println!("Failed to find Contracts::CodeStored Event"),
+            }
+        },
+        Err(err) => println!("Error: {}", err)
+    }
+}