Add support for Github's pages

In order to add [Github pages](http://pages.github.com) support to coleslaw, the code needs to
1. Generate a file named `CNAME` under the root of the generated site, whose content is the domain name provided in `.coleslawrc` but without the scheme (e.g http://, https://, etc.)
2. Github pages does not support symlinking; therefore, instead of making `index.html` a symlink to `1.html`, `1.html` is copied as `index.html`

This patch add [puri](http://puri.b9.com/) as another dependency to coleslaw. This library helps extract the provided domain in `.coleslawrc` robustly.