Add the ability to render README from the package root

[tfoote]

As we're adding index.ros.org support which will render README files from packages. It would be good to support rendering READMEs for packages on the wiki too. This will allow the package README to be authoritative and rendered in both locations.

To make this happen the package header could be extended to remotely load the README from github's raw API if it exists.

This would complement ros-infrastructure/rosindex#103