Should use unicode_literals

[Caio Begotti]

In order to be as Python3 compatible (with the dependency on paramiko excepted) we should import and use unicode_literals.

From https://github.com/capomastro/capomastro/issues/25

[Daniel Manrique (roadmr)]

This step consists in:

    Adding from __future__ import unicode_literals at the top of your Python modules – it’s best to put it in each and every module, otherwise you’ll keep checking the top of your files to see which mode is in effect;
    Removing the u prefix before unicode strings;
    Adding a b prefix before bytestrings.

straight from:

http://django.readthedocs.org/en/latest/topics/python3.html

However, there are more items to be considered if we want to be Python3-compatible. I'll set this for future if someone wants to have a go at it, but I suspect it'll be deferred until we go ahead with full Python3-compatibility work.