Added 'end' attribute to ReservedInstance

Hi. I've happened upon #2757 and decided to fix it. 

I was a bit unsure what the right approach to testing this was. There was no existing test for the ``EC2Connection.get_all_reserved_instances`` call. In the end I've mimicked [one of the latest added tests](https://github.com/boto/boto/blob/develop/tests/unit/emr/test_connection.py) - is this test how the tests should be written currently?