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

src/rpc.rs

@@ -170,9 +170,7 @@ impl<T: System> Rpc<T> {
 
     /// Get a block hash of the latest finalized block
     pub fn finalized_head(&self) -> impl Future<Item = T::Hash, Error = Error> {
-        self.chain
-            .finalized_head()
-            .map_err(Into::into)
+        self.chain.finalized_head().map_err(Into::into)
     }
 
     /// Get a Block