core, plugin XEP-0115: discovery refactoring:

- hashing algorithm of XEP-0115 has been including in core
    - our own hash is still calculated by XEP-0115 and can be regenerated with XEP_0115.recalculateHash
    - old discovery methods have been removed. Now the following methods are used:
        - hasFeature: tell if a feature is available for an entity
        - getDiscoInfos: self explaining
        - getDiscoItems: self explaining
        - findServiceEntities: return all available items of an entity which given (category, type)
        - findFeaturesSet: search for a set of features in entity + entity's items
      all these methods are asynchronous, and manage cache automatically
    - XEP-0115 manage in a better way hashes, and now use a trigger for presence instead of monkey patch
    - new FeatureNotFound exception, when we want to do something which is not available
    - refactored client initialisation sequence, removed client.initialized Deferred
    - added constant APP_URL
    - test_plugin_xep_0033.py has been temporarly deactivated, the time to adapt it
    - lot of cleaning

src/core/exceptions.py

@@ -74,6 +74,10 @@ class InternalError(Exception):
     pass
 
 
+class FeatureNotFound(Exception): # a disco feature/identity which is needed is not present
+    pass
+
+
 class BridgeInitError(Exception):
     pass