Status badge from package page (WIP)

Adds a GoDoc badge #107 from the package page #115.

* [x] Badge in the footer of packages that links to the ?status page
* [x] Markdown and HTML to copy to a README file (not sure about encoding, quote marks, etc.)
* [x] Generates token for status.png (I imagine this should be cached somewhere?)
* [ ] Dynamic badge text [Buckler](http://buckler.repl.ca/)
* [ ] README/code Contains token (store in Redis?)
* [ ] Search rankings and indicator when token is present