plugin file, XEP-0234: registering is now done by class + use of async: - instead of registering a callback, a file sending manager now register itself and must implement some well known method (`fileSend`, `canHandleFileSend`) and optionally a `name` attribute - `utils.asDeferred` is now used for callbacks, so all type of methods including coroutines can be used. - feature checking is now handled by `canHandleFileSend` method instead of simple namespace check, this allows to use a method when namespace can't be checked (this is the case when a file is sent to a bare jid with jingle)