Add a way to refresh a collection

The issue was this:

>>> applications = client.applications
>>> myapp = applications[0]
>>> myapp.delete()
>>> for app in client.applications: print app.name  # The deleted app was still there

This pull request adds a method to the collection called refresh:

>>> client.applications.refresh()
>>> for app in client.applications: print app.name # The deleted app will *not* show up


Also added a tiny tweak to tox, to make it easier to use when debugging.