add makefile for simpler compilation

Makefile

@@ -0,0 +1,15 @@
+CARGO=cargo
+BACKENDS=backend_piston,backend_gilrs
+
+all:
+	cargo build --features=${BACKENDS}
+
+test:
+	cargo test --features=${BACKENDS}
+
+doc:
+	cargo doc --features=${BACKENDS}
+
+clean:
+	cargo clean
+