Py23 unicode

[ChristopherChudzicki]

Background: https://portingguide.readthedocs.io/en/latest/strings.html, merge after #200

The goal of this PR is to make our library:

• use unicode strings internally
• allow Python 2 end-users to use unicode or str. This is achieved by the text_string voluptuous validator

Other notes:

• Almost all mitxgraders/ files use from __future__ import unicode_literals
  • The except is __init__ files, since __all__ cannot contain unicode in Python 2
• No files in tests/ use unicode_literals future import (since our end-users can't use future imports on edX)

[jolyonb]

I'll get you to rebase this after #200 is merged. Then I'll review ;-)

[jolyonb]

Was just going to ask you to close this and reopen a clean one ;-)