Add "Report An Issue" action in developer documentation toc

This adds a "Report An Issue" entry in devel docs table of content, so people reading the documentation can easily report any mistake / typo on GitHub anywhere on the page. If javascript is enabled, the GitHub issue description is automatically filled with current browser location url.

Edit: Thanks to @harding for suggesting GitHub REST API idea.

![capture du 2014-05-19 12 03 54](https://cloud.githubusercontent.com/assets/3578089/3016544/48a8128c-df6f-11e3-8cf7-914de06b2f61.png)