Endroid doesn't handle unicode

[Phil Connell]

Endroid currently uses byte strings (i.e. Python2 str()) everywhere rather than unicode.

This leads to ridiculous code like *encoding* a string just fetched from the database before it's used elsewhere, e.g.
   users_blocked = users_blocked.encode('ascii')
in
   https://code.launchpad.net/~jamesh-f/endroid/smsblock/+merge/232910

Changes that should be made:
  - from __future__ import unicode_literals everywhere
  - Decode from byte strings at the edges of endroid core (e.g. when receiving a message, reading from the database, ...)
  - Plugins should never see byte strings.