Append "/" to urlpath if not present

If urlpath doesn't end in a "/", then Handle treats the path as rooted.  It will serve the directory urlpath but not any of the files or subdirectories contained within, unless one of them is "index.html".

See the pattern handling description at [http://golang.org/pkg/net/http/#ServeMux]
Merge pull request #1 from kalmduck/patch-1

Append "/" to urlpath if not present