Adapt to minor upstream API changes; clean up.

Include standard libraries from GOROOT in Go package list.
Use req rather than r for *http.Request.
Move WriteGitHubFlavoredMarkdown() into github.com/shurcooL/go/u/u1.
Remove obsoleted helper funcs.
Add tool to show the importers of current package via godoc.org API.

Thank you to @garyburd and @golang for making this possible.