1) Absolute paths without a hostname were being rewritten relative to
   the current page, rather than relative to the current host.
   e.g. When viewing /some/page.html, it would rewrite:
       /other/page.html => proxyPrepend/some/other/page.html
   Instead, it should rewrite:
       /other/page.html => proxyPrepend/other/page.html

2) Trailing slashes were being stripped from all rewritten URLs.
   This is because path.Join() always calls path.Clean() as well:
   http://golang.org/pkg/path/#Join