`io` module is present in Python 2.6

First import for `StringIO` must be from `io` module because this is the future. Fallback from `cStringIO` and finally from `StringIO` must be for Python 2.5 compatibility. If Python 2.5 is not supported then this `try .. except` block must be removed and only the import from `io` module should be left here.
Merge pull request #10 from vstoykov/patch-1

`io` module is present in Python 2.6
Merge remote-tracking branch 'upstream/master' into attachments

Conflicts:
	django_mailgun/__init__.py