Use basestring, and allow shim for Python3 later.
Merge branch 'bugfix/use-basestring' into feature/debian-binary-integration
Switch using `str` for `basestr`

 Python 2.7 uses `unicode' by default, which isn't a subclass of
 `str'. This also allows us to switch out the str_type later, for
  when Python 3 support is added.
Merge pull request #8 from paultag/bugfix/use-basestring

Bugfix/use basestring