Ensure Python 2.7 compliance - fix #13

[lukpueh]

This is a rather huge PR but it has a lot of similar changes all over the place. The changes are thematically grouped in separate commits:

• Use six to provide consistency over renamed packages (getcwd, xrange, xmlrpc)
• Add print_function pseudo module to provide fully-fledged print functions
• Add unicode_literals pseudo module to make all string literals unicode without explicit markup (like u'Hellü')
• Use open from io instead of builtin to provide consistency in terms of functionality
• Use canonicaljson to utf-8 encode JSON strings and write the to disk in binary mode. They can be read in text mode with the utf-8 encoding specified.
• Update README accordingly
• Add according language classifiers to setup.py

Tested on Python 2.7.10 and Python 3.5.1 using demo.

Note on __future__: Python's migration guide recommends to always add a certain set of __future__ import satements to new Python modules. I only added them where needed. Please keep in mind that they might need to be added as the code changes.

[awwad]

Long term, I don't want to add the canonicaljson lib as a dependency, so I'm going to update the code that generates JSON to produce canonical json. TUF does something similar (You can find it here in tuf/client/repository_lib.py).

For now, I'll just use canonicaljson as added in this PR.