[omemo] Refactor Plugin

- Adapt to nbxmpp supporting OMEMO
- Move python-axolotl code into backend folder

omemo/backend/db_helpers.py

@@ -0,0 +1,15 @@
+''' Database helper functions '''
+
+
+def table_exists(db, name):
+    """ Check if the specified table exists in the db. """
+
+    query = """ SELECT name FROM sqlite_master
+            WHERE type='table' AND name=?;
+        """
+    return db.execute(query, (name, )).fetchone() is not None
+
+
+def user_version(db):
+    """ Return the value of PRAGMA user_version. """
+    return db.execute('PRAGMA user_version').fetchone()[0]