zpm: use python-swiftclient (fixes #31)

We now use python-swiftclient instead of our own miniswift library.
This required some small changes:

* The new client will automatically notice when it needs to login, so
  the explicit calls to authenticate have been removed.

* Unlike miniswift, the put_object in python-swiftclient takes
  separate arguments for the container and object name, so the upload
  paths need to be split on the first '/'. This has been tested by
  hand since we don't have mocks for this part of the code yet.
miniswift: remove unused library