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.
It also supports using the 'named' selector for findAll call, which was
not the case in the original implementation.
Renamed the exact and partial named selectors
Added a BC layer for the named selector in the handler

This BC layer will ensure that code using the SelectorsHandler directly
will not break when trying to access the named selector. However, it will
trigger a deprecation notice.
Merge pull request #451 from stof/prefer-exact-matches

Prefer exact matches in the named selector