RapidShare external files backend

As already announced on the mailing list (http://mail.kde.org/pipermail/owncloud/2013-February/007643.html), I've created a storage backend for the One-Click Hoster RapidShare.

The backend is designed to do block-level operations on files stored on the RapidShare servers (i.e. files don't have to be fully transmitted to the local server to edit them). This was the major criterion for selecting this specific provider. The performance is nevertheless acceptable. The backend supports free as well as paid (RapidPro) users and determines the best way to access files automatically.

As soon as you consider my contribution as acceptable for inclusion into the core, I will sign the Contributor Agreement (as it clearly makes no sense to sign it if the contribution is never to be included).