Specifically the methods related to requesting an upload slot and uploading a file.
Also show a progress indicator while a file is being uploaded.
Updates #161
* Use Promises instead of callbacks
* Update to latest (Last Call) version of XEP-0363
* Move non-view specific methods to models instead
* Add more tests
updates #161
Previously we kept all entities and their items (which are also
instances of _converse.DiscoEntity) in a flat array.
Instead, we should have a tree-like structure where items are stored
on the relevant entity (and recursively on other items).
