Fixed blog_url not returning & rake setup_github_pages failing

When executing `rake setup_github_pages`, an error tells that `url` don't exist. The reference to that variable has been changed to a call to `blog_url`.

The function `blog_url` was not returning anything, failing to properly write the `_config.yml` file. It now does return what should have.
Fixed blog_url when user contains uppercase letters
Made blog_url becalled just once...

...and stored in the `url` variable.
Octopress init
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