Add Tox environments for running linters and include one in the default env list. Add a Tox environment for running leak tests and one for doing both, but do not run them by default. Unify the Travis and Tox configurations, and make Travis run both leak and lint tests. Switch the leak test from using gettotalrefcount (only available in debug builds) to using len(gc.get_objects()); while this is not exactly the same thing, it make be close enough, and it's avaialble to every Python developer unlike gettotalrefcount.