add Python3 compatibility

This commit adds support for Python3. In Django 1.5 `force_text` was introduced to replace `force_unicode` only available under Python2.
This commit uses `force_text` whenever possible and fallback to `force_unicode` when not available. This makes django-epiceditor compatible
for both Python2 and Python3 and maintains support for Django >= 1.4.
Bump version number to 0.2.3