Return 304 code during a hour

- Inspired in https://github.com/widmogrod/zf2-semi-http-cache
- Add last modified and cache control

Actually If I can get the path (like ```$assetManager->getAssetPath()```) I'll get the modified time using ```filemtime()```, it'll compare and return right LastModified... And the cache will work better, not only for a hour.