Fix undo registering when pasting and dragging

See: https://github.com/learndot/sproutcore-wysiwyg/issues/22
Adds filtering for bonkers MS Office paste jobs.
Merge pull request #25 from dcporter/dammit_word

Adds filtering for bonkers MS Office paste jobs.
Merge pull request #24 from nicolasbadia/master

Fix undo registering when pasting and dragging