Add new extension path to match Laravel 4.3

Referring to this request https://github.com/orchestral/extension/issues/48

Added new ```Extension``` directory inside the ```app``` directory as one of the default location for new extensions. In this way we can use the ```App\Extension\NewExtension``` easily without the hassle of updating the ```composer.json``` file.

Since that previously one of the default path was ```app\``` i think it should be removed from the default path due to the new structure of 4.3.

Signed-off-by: Ahmad Shah Hafizan Hamidin <ahmadshahhafizan@gmail.com>
Let's just use `App\Providers\ExtensionServiceProvider` since we're maintaining orchestra/platform.