Fix include paths

This pull requests "fixes" the include paths used on libRocket so all paths are "absolute" from the Rocket root. I needed this because the build system in my engine flattens the source code into a single folder and the compiler was having trouble finding the headers. It's a trivial but extensive change, I leave it up to you if you think it's worth incorporating.
I mistakenly added the readme.md change I made to mark my repo as having changes for my engine, so you may want to remove that :)