Add rabbitmq helpers

This PR adds the `test_helpers.rabbit.RabbitMqFixture` class which makes it easy to test services that use Rabbit MQ.  The class and its usage is best described [by its docstring](https://github.com/aweber/test-helpers/blob/3d88446da418fc037753202cc483b30646b6f2d4/test_helpers/rabbit.py#L15-L44).

I also added `test_helpers.mixins.EnvironmentMixin` which is useful if you need to manipulate `os.environ` within your test cases.  It provides some helpers that ensure that tests remain isolated.  I needed this to test the RabbitMQ fixture class and figured that it probably has some redeeming value onto itself.