Prefer exact matches in the named selector

This uses the approach implemented by @bevanr in #386 but rebuilds it on top of the latest Mink where the NamedSelector was changed a lot. Rebuilding from scratch was easier than trying to rebase given the huge changes done in these parts of Mink recently.
It also supports using the 'named' selector for findAll call, which was not the case in the original implementation.

Replaces #386