Feature collection upload async

Handle asynchronous file upload for collection_upload form type.

This is ready to merge.

TODO:
 [] Test and review: @loostro, if you want to give me some feedbacks, don't hesitate
 [] Add documentation
 [] Create Gaufrette File Storage Adapter (not for this PR)


Please, do not hesitate to comment

(https://github.com/sescandell/CollectionUploadSample is up to date with this functionnality).