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


Yep, this comment was of my own misconception. I meant to say that we don't deal with multi-columns in vertical writing mode.