Added SpiderOak

SpiderOak provides an easy, secure and consolidated free online backup,
storage, access, sharing & sync tool for Windows, Mac OS X, and Linux
(Ubuntu, Debian & Fedora). SpiderOak offers a different approach to
online backup, synchronization, and sharing. This difference is not
just measured in our zero-knowledge privacy policy - the first one ever
employed in this setting - but also in our flexible design in handling
data from all platforms (Mac, Windows, Linux) and locations (external
drives, network volumes, USB keys) in one centralized account.