Moving extension to be in line with the symfony standards

The extension naming should follow the
[docs](http://symfony.com/doc/current/cookbook/bundles/extension.html#creating-an-extension-class)

> If you do choose to expose a semantic configuration for your bundle,
> you'll first need to create a new "Extension" class, which will handle
> the process. This class should live in the DependencyInjection
> directory of your bundle and its name should be constructed by
> replacing the Bundle suffix of the Bundle class name with Extension.
> For example, the Extension class of AcmeHelloBundle would be called
> AcmeHelloExtension: