GitLab

Fixes #111

So callback methods don’t need to catch it they don’t need to handle
it specially

This method returns the disco info and vcard of a MUC

- Add set_avatar()

Module provides a decorator which wraps a generator in a Task
The Task iterates the generator until it ends and processes the yielded values.

Example:
```
@iq_request_task
request_something(self, jid):
    task = yield

    response = yield get_request_iq()
    result = process(response)

    if is_error_result(result):
        raise StanzaError(result)
    yield result
```

Fixes #82

- Add validation for nicknames
- Bookmarks2: Parse password element