Detection improvements

This builds on my branch *issue-33*, so the first two commits are the ones you already know from there.
Therefore, if you accept this pull request, we can cancel the other one.

This includes a couple of improvements regarding detection of clickable elements:
- Detection of elements with ARIA markup. This makes important sites like GMail usable.
- Detection of elements by cursor style (elements having cursor style set to 'pointer').

Also, implementing these improvements I stumped on a couple of issues, whose solution are also included:
- Some elements were detected but click simulation didn't work on them.
- Some link hints were shown for elements that were shadowed by another element (popup menus and the like).

Please see full message commits for more details.

And by last, one question: if you accept this pull request, will you merge this into your master, or any other branch of your own? I ask this because I'm thinking about more improvements, and I would like to be able to base subsequent pull requests on a branch of yours that already has previously accepted pull requests, so that new pull requests only contain new commits. If you prefer working another way, please explain. Thanks.