gpaste-client: add async ctor

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>
Merge branch 'wip/async-dbus'

* wip/async-dbus:
  gpaste-client: add async ctor
update TODO

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>