Fixed bug preventing plugin configurations from being read

In `api.load_configuration_object`: change 'r' to 'rb' for pickled file (otherwise the read was failing since `save_configuration_object` writes as 'wb').

Otherwise settings were not saved persistently in these plugins which call that function (well, they were saved but not loaded properly so set back to default):

* notification area
* hamster
* untouched tasks
* urgency color
* export
* task reaper