Allow downloading CSL files from Github

#4395 

``admin/config/content/biblio/citeproc``

![biblio_settings___site-install](https://f.cloud.github.com/assets/125707/1672436/04941048-5cd2-11e3-81bd-2405403eea6f.png)

Notes about this PR:

* The Biblio we have is now treated as custom, that's why you can see the changed files being under the ``temporary/`` folder.
* PR includes the original patch, just because that's how we did until now, it probably doesn't have much meaning. It also deleted a previous patch file, that is overridden by this new one.
* The important change in the PR is that instead of using an outdated Github API library, we simply use ``drupal_http_request()`` to get the CSL files.