Support deleting all messages of a private chat

Fixes #20 (closed).

briar_wrapper/models/private_chat.py

@@ -63,3 +63,16 @@ class PrivateChat(Model):
         url = urljoin(BASE_HTTP_URL,
                       self._API_ENDPOINT + str(self._contact_id) + "/read")
         requests.post(url, headers=self._headers, json={"messageId": message_id})
+
+    def delete_all_messages(self):
+        # pylint: disable=line-too-long
+        """
+        Deletes all messages
+
+        [Upstream documentation](https://code.briarproject.org/briar/briar/-/blob/master/briar-headless/README.md#deleting-all-private-messages)
+
+        .. versionadded:: 0.0.5
+        """
+        url = urljoin(BASE_HTTP_URL,
+                      self._API_ENDPOINT + str(self._contact_id) + "/all")
+        requests.delete(url, headers=self._headers)