Add base class. Make to_json available to all of the existing models.

flask_discord/models/base.py

@@ -0,0 +1,15 @@
+from abc import ABC
+
+
+class DiscordModelsBase(ABC):
+
+    def to_json(self):
+        """A utility method which returns raw payload object as it was received from discord.
+
+        Returns
+        -------
+        dict
+            A dict representing raw payload object received from discord.
+
+        """
+        return self._payload