Fix #84: implement blocklist retrieval

Also fix some incompatibility issues with XEP-0191

Signed-off-by: Daniele Ricci <daniele.athome@gmail.com>
Bot: request blocklist and block user commands

Signed-off-by: Daniele Ricci <daniele.athome@gmail.com>
Release subscriptions correctly (closes #85)

Signed-off-by: Daniele Ricci <daniele.athome@gmail.com>