If a file named index.html exists in a directory, or root, it will be
rendered as if ugly urls are turned on. This allows for top level
content to not need a supporting layout file and content in content.
This change should not affect anyone who is using the perscribed way.
I also cleaned up a bunch of one off functions in site.go.
filepath was used inconsistently throughout the hugolib. With the
introduction of source and target modules, all path are normalized to
"/". This simplifies the processing of paths. It does mean that
contributors need to be aware of using path/filepath in any module other
than source or target is not recommended. The current exception is
hugolib/config.go
