Add validator_node, restructure binaries, gRPC.

This is a massive commit which restructures the workspace, adds a very
basic, untested, validator client and some very basic, non-functioning
gRPC endpoints to the beacon-node.

protos/build.rs

@@ -0,0 +1,8 @@
+extern crate protoc_grpcio;
+
+fn main() {
+    let proto_root = "src/";
+    println!("cargo:rerun-if-changed={}", proto_root);
+    protoc_grpcio::compile_grpc_protos(&["services.proto"], &[proto_root], &proto_root)
+        .expect("Failed to compile gRPC definitions!");
+}