new styles and code for right panel toggle

This revises the code and styles for the right panel toggle.

It positions the toggle above the sidebar, inline with the breadcrumbs.  This prevents it from interfering with a right aligned table of contents.  The toggle doesn't move when clicked.  It also expands and contracts when hovered over.  It fixes some bugs in the code as well.

I'm not entirely sure about the discoverability of this, but it doesn't seem like such an essential feature that it being slightly hidden is a problem.

Some screenshots:

Default:
![visible](https://cloud.githubusercontent.com/assets/50029/4444919/dfd4ea20-47f4-11e4-93a6-8693c30651e0.png)
Hovering over button:
![visible-hover](https://cloud.githubusercontent.com/assets/50029/4444923/e639dfb0-47f4-11e4-9330-32ff5b653e31.png)
Hidden:
![hidden](https://cloud.githubusercontent.com/assets/50029/4444925/eaf843fc-47f4-11e4-8719-4d340baa78b1.png)
Hovering over button when hidden:
![hidden-hover](https://cloud.githubusercontent.com/assets/50029/4444932/f6325abe-47f4-11e4-88e0-42a4f37c6beb.png)