Add support for CommonJS style workflows.

This allows ES6/commonjs workflows to use operative without adding it to the global scope. This also satisfies issue padolsey#39 padolsey#39

src/operative.js

@@ -127,5 +127,9 @@
 	};
 
 	// Expose:
-	window.operative = operative;
+	if (typeof module !== 'undefined' && typeof module.exports !== 'undefined')
+		module.exports = operative;
+	} else {
+		window.operative = operative;
+	}
 })();