Resolve plugin dependency chains with XEP-0115.

The post_init() system can only reliably handle a single layer
of dependencies between plugins, but PEP plugins with XEP-0115
exceed that limit and plugins can be post_init'ed out of order. To
resolve this, we will special case XEP-0115 to be post_init'ed
first until the new plugin system with dependency tracking is
stable.

sleekxmpp/plugins/__init__.py

@@ -27,7 +27,9 @@ __all__ = [
     'xep_0086',  # Legacy Error Codes
     'xep_0092',  # Software Version
     'xep_0115',  # Entity Capabilities
+    'xep_0118',  # User Tune
     'xep_0128',  # Extended Service Discovery
+    'xep_0163',  # Personal Eventing Protocol
     'xep_0199',  # Ping
     'xep_0202',  # Entity Time
     'xep_0203',  # Delayed Delivery