Ability to alter the document history

I think we do need to be able to alter the document history, for removing spam.

I know it is not nice to alter history, but:

• older versions are always accessible (in the new skin not for simple users, nor for search engines, but who know what the future holds for us)
• keeping links to bad sites will both increase the target sites' popularity and decrease it for the spammed wiki
• it's not good for children

Things to remember:

• It should really change the history, not like the current rollback, which just adds a new version
• It is not OK for anybody to be able to do this, but only administrators
• It should respect the Recycle Bin timing policy mentioned in XWIKI-543
• Should we completely remove a version, leaving a gap in the history, or just replace the content with "this version was deleted"?