Add support for Python 3

First of all, thanks for a great Django app @nebstrebor. I've used in several project so far and am really happy with it.

Now to the PR: I've started playing around with Python 3 and Django. In my little project I'd like to use your shortuuid field but importing it fails due to some incompatibilities with Python 3:

1. The ``try...except`` uses the old-style exception assignment that is not support in Python 3 anymore. Since you are not using the ``e`` variable anyway, I have simply removed it.
2. You are using an implicit relative import in ``shortuuidfield.__init__.py`` which doesn't work in Python 3. Only explicit relative imports are supported. I have updated the import statement and added another import to enforce the explicit behaviour in Python 2.x. The [Python wiki](https://wiki.python.org/moin/PortingPythonToPy3k#relative-imports) has some more details on the relative imports if you are curious.

Looking forward to your feedback.