Add support for humans.txt/author relations.

This adds support for [humans.txt](http://humanstxt.org/). We should discuss whether or not adding this is a good idea. The humans.txt website specifies that the `humans.txt` file should be located in the root and be linked to using `<link rel="author" type="text/plain" href="humans.txt">`. This change supports that syntax but still rewrites the asset to be in the static directory. I'm not sure it is a good idea to move it to the site root as that will mess up caching. On the other hand, the `humans.txt` file won't be requested often. Discuss.