Fixed Mouse Up keyword attempting to click and hold one more time before release

I've almost fixed all failing tests, please take a look at the details below:

1. Fixed Mouse Up keyword attempting to `click_and_hold` before `release` therefore emitting `mousedown` event;
2. Fixed `Unselect Non-Existing Item From List` test post-processing `LOG` command failing with wrong keyword index;
3. Refactored `Mouse Down On Link` test to be clearer;
4. Updated `Click Element At Coordinates` test with more "expected" coordinates.

Also I would like you to take a look at `Click Element At Coordinates` test one more time. It seems to me that selenium doesn't perform `move_to_element.move_by_offset.click` action chain well. I've submitted the issue on [google code](http://code.google.com/p/selenium/issues/detail?id=6370).