Integrate unaccent lookups in django.contrib.postgres

[Thomas C]

After the introduction of the Lookup API in Django 1.7, I believe that django.contrib.postgres could benefit from some custom lookups based on the unaccent extension of PostgreSQL (see ​http://www.postgresql.org/docs/9.3/static/unaccent.html). That kind of feature would be very helpful for a lot of people in latin countries, and the existing solutions are not very clean (and do not make use of the new API).

The way I see it, unaccent would be a sub-application within django.contrib.postgres which should be enabled for the lookups to be active (that requires postgres >= 9.0).