GlobalTOC Macro

See https://github.com/gollum/gollum/issues/921#issuecomment-67859884.

`<<GlobalTOC(Pages in this Wiki)>>` renders as:

![screen shot 2014-12-22 at 20 57 43](https://cloud.githubusercontent.com/assets/147111/5530259/2053ebb2-8a1e-11e4-8af8-bb6f336d2454.png)

Feedback on the layout appreciated -- I used the ordinary TOC's layout so this can easily be included in sidebars, too.