Modify GitHub pages domain

Now this page is served as `rack.github.io`.
* https://github.com/blog/1452-new-github-pages-domain-github-io
Merge pull request #572 from tricknotes/modify-github-pages-domain

Modify GitHub pages domain
add LINK and UNLINK methods
Merge pull request #575 from jamiehodge/link-unlink

add LINK and UNLINK methods to method override
add #link? and #unlink?
Merge pull request #578 from jamiehodge/link-unlink

add #link? and #unlink?
html escape detail for error message
Merge pull request #579 from nealharris/escape-error-message-in-rack-showstatus

html escape detail for error message
Add Response#unauthorized? helper

This helper should come in handy when you want to test the authorization
in your application.
Merge pull request #580 from sikachu/unauthorized_helper

Add Response#unauthorized? helper