Custom field rendering with django-bootstrap3

If it is installed use django-bootstrap3 for field rendering. This
fixes rendering on chrome / webkit based browsers.

To use the provided class you need to install django-bootstrap3 and
add this in settings:

DAB_FIELD_RENDERER = 'django_admin_bootstrapped.renderers.BootstrapFieldRenderer'

Fix #152
Bump release to 2.3.0