tox: drop support for Python 3.2

Python version 3.0 to 3.2 had no support for u'...' Unicode literals.
The support was added again in Python 3.3 to make it easier to port
code from Python 2.x.

The lack of u'...' support has not been a direct problem for us, we
would like to use python-swiftclient instead of our own miniswift
module (#31). However, python-swiftclient only supports Python 3.3 and
later. More precisely, python-swiftclient tries to import simplejson
and this fails with a SyntaxError on Python 3.2.

The simplejson project will probably not support Python 3.2 anytime
soon:

  https://github.com/simplejson/simplejson/issues/66

We will therefore also drop support for Python 3.2 for now. We will
work with the upstream projects to see if it is easy to restore Python
3.2 support. The first pull request for one of our dependencies has
been positively received:

  https://github.com/kennethreitz/requests/pull/2064

Pip is also affected by this problem and an issue has been created:

  https://github.com/pypa/pip/issues/1839
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
Merge pull request #126 from mgeisler/use-python-swiftclient

Use python swiftclient