Fixed setup_github_pages failing and not properly writing url to _config.yml

This PR fixes the following bugs:

+ `blog_url` wasn't returning anything, so the `url` field of `_config.yml` was being emptied.
+ `blog_url` was writing the URL with uppercase letters (e.g., `http://Darkhogg.github.io`). Username is now downcased (project is not, I'm not sure if that would be required)
+ Rake task `setup_github_pages` was trying to print the `url` variable, which is undefined. It now calls `blog_url` again.

I did not search through issues for all these bugs so I don't know if this would fix some open issue.