Remove dependency on Python 2.7 by not using ZipFile context manager

This resolves errors with the CKAN datastorer when running under Python 2.6 by not using ZipFile context manager (only available in Python 2.7)

e.g.
> File "messytables/messytables/zip.py", line 17, in from_fileobj
>    z = zipfile.ZipFile(fileobj, 'r')
> AttributeError: ZipFile instance has no attribute '__exit__'

Inspired by patches from other projects like https://github.com/Diaoul/subliminal/commit/b24af1732619c9174d0e912df9101ea2240d71fb
Includes a naive Zip file test case.