Unicode strings throughout.

[tomchristie]

Use from __future__ import unicode_strings everywhere.
Check __repr__ returns correct type under both python 2 and 3.

[tomchristie]

Note that we don't use this in the tests as it makes testing repr and str representations almost impossible to do in a py2<->py3 way (as we have strings with ...u'something'... that we're comparing against.