Add node/fork endpoint to HTTP API, tidy

beacon_node/http_server/src/key.rs

@@ -0,0 +1,12 @@
+use beacon_chain::{BeaconChain, BeaconChainTypes};
+use iron::typemap::Key;
+use std::marker::PhantomData;
+use std::sync::Arc;
+
+pub struct BeaconChainKey<T> {
+    _phantom: PhantomData<T>,
+}
+
+impl<T: BeaconChainTypes + 'static> Key for BeaconChainKey<T> {
+    type Value = Arc<BeaconChain<T>>;
+}