Like and ILike lookup (Ticket #17473)

Add a new lookup to querysets. Now you can do something like this:
    MyModel.objects.filter(field_name\__like='xx%YY%zz')
    MyModel.objects.filter(field_name\__ilike='xx%yy%zz')
    User.objects.filter(username\__ like='a%in')

It is more quick that something like this, and more readable:
    MyModel.objects.filter(field_name\__regex='^xx.*yy.*zz$')