DziTileSource: avoid changing relative paths

For cases where DZI files aren't explicitly configured with a tilesUrl,
the logic for generating a URL from the DZI source URL would inject an
extra leading `/` which would either cause an unnecessary redirect or
break depending on whether the webserver in use attempts to normalize
`//` to `/`.

This change also removes some URL processing logic which duplicates the
browser's default behaviour
Merge pull request #56 from acdha/dzitilesource-relative-paths

DziTileSource: avoid changing relative paths