Upgrade Peewee to latest 3.x

This requires a number of small changes in the data model code, as well as additional testing.

test/test_endpoints.py

@@ -161,7 +161,7 @@ class BuildLogsTestCase(EndpointTestCase):
     logs = ['log1', 'log2']
     with patch('endpoints.web.build_logs.get_log_entries', return_value=(None, logs) ):
       resp = self.getResponse('web.buildlogs', build_uuid=self.build_uuid, expected_code=200)
-      self.assertEquals('{"logs":["log1","log2"]}\n', resp)
+      self.assertEquals({"logs": logs}, py_json.loads(resp))
 
 class ArchivedLogsTestCase(EndpointTestCase):
   build_uuid = 'deadpork-dead-pork-dead-porkdeadpork'