TinyMCE clean up on paste

This PR adds "clean up on paste" option to TinyMCE settings. I think this is very basic and usefull settings.

For example, if you copy just 4 short string from MS Word document (attached to tracker item) to TinyMCE, then you will get plenty of garbage code: http://pastebin.com/7S43SQhC

If you apply PR and enable "clean up on paste" option, then when copying the same document to TinyMCE you will get this: http://pastebin.com/cwzE8Tqn

<h3>Tracker item</h3>
http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=33796&start=0