Refactors bindinds to use async workers

This fixes concurrency issues reported in #189.

References:
- https://github.com/nodejs/nan#asynchronous-work-helpers
- https://github.com/nodejs/nan/blob/master/doc/asyncworker.md#api_nan_async_worker
- https://github.com/nodejs/nan/blob/master/doc/asyncworker.md#api_nan_async_queue_worker

Author: TheMarex

src/json_v8_renderer.hpp

@@ -92,7 +92,6 @@ inline void renderToV8(v8::Local<v8::Value> &out, const osrm::json::Object &obje
     osrm::json::Value value = object;
     mapbox::util::apply_visitor(V8Renderer(out), value);
 }
-
 }
 
 #endif // JSON_V8_RENDERER_HPP