Visibility computations by client rectangles, vertical writing mode (column layout) #87

Fixes issue #87.

Tested: 
- Media overlay playback and page progression
- Navigating to element CFI using reader.openSpineItemElementCfi (bookmarking)
- Navigating to element Id using reader.openSpineItemElementId (TOC element id linking)

Tested under:
- Chrome 37
- Safari 7
- iOS 7.1.1 (iPad)

EPUBs used:
RTL: https://code.google.com/p/epub-samples/downloads/detail?name=kusamakura-japanese-vertical-writing-20121124.epub&can=2&q=
LTR: https://drive.google.com/file/d/0BwtT87vbGnJuZFdoZVFWSnNaelU/edit?usp=sharing


There actually are CSS columns in vertical writing mode, but the entire iframe / visible rendering viewport is filled with a single column (double page side-by-side / synthetic spread is never enabled in vertical writing mode), and the columns are stacked vertically (from top to bottom) instead of being laid in the classic horizontal RTL or LTR direction.