Revisions: Re-work how tick marks and tooltips are aligned. IE fixes.

* Pixel-based alignment of tooltips.
* Bottom-based alignment, so arrow lines up using CSS only.
* Better RTL styles (mostly mirror-imaged).
* Better RTL calculations in `revisions.js` (less logic).
* Better IE support.

See #24736. Props markjaquith, adamsilverstein, ocean90.

git-svn-id: http://core.svn.wordpress.org/trunk@24751 1a063a9b-81f0-0310-95a4-ce76da25c4cd