Add 'detach' scope feature to Asset storage

A detach asset :
 - must have his parent scope set to 'none',
 - don't override a asset with the same name from classic scope

The scope 'none' don't exists, it's just a parent scope
add 'Asset Loader' features

- provide public api to define a new Loader of Asset
- add 'Default Loader' based of 'dandelion/dandelion.properties' (key ' assetDefaultLoader') or by default on the 'Json Loader'