Support Pathname

These two commits allow client code to use Pathnames when working with Rake.  The first commit allows transparent use of Pathnames in FileList, file/directory tasks, and as task prerequisites.  The second adds the `ext` and `pathname` extension methods to `Pathname`.

I believe I caught all of the places where Pathnames might be used, but it is possible that I missed something.

The basic philosophy I followed was to convert incoming Pathnames to strings as soon as possible, and allow Rake to work with strings as it always has.

I'd like to find a way to have the `ext` and `pathmap` extensions defined automatically if the client code requires 'pathmap'; I didn't want to make Rake require `pathmap` itself and force it into client code.