Add the possibility to filter a field 'accent insensitively'

Until Django 1.8, there is no simple way to filter a field 'accent insensitively'.
An example occurred in the Vulnerability application, where some authors had accents in their names and it was not possible to find those names if they were written with wrong accents in the search field.
We need a custom textual field that could be filtered both 'case insensitively' and 'accent insensitively', leveraging a custom postgres operator able to perform the search.