Sanitize the output and text cells of published worksheets.
Add nofollow to html scrubbing.  Thanks to Volker Braun for bringing it up.
Don't scrub html twice.
Remove the remainder of the whatwg unsafe tags.

See https://groups.google.com/forum/?fromgroups=#!msg/sage-devel/ze5PZc1Wyjk/sjc-vkYbkUIJ for a discussion.
Merge pull request #100 from jasongrout/sanitize-published

Sanitize the output and text cells of published worksheets.