Hg remove empty ps

This fixed the problem of <b> tags wrapping <p> tags and creating invalid HTML when pasting from Google Docs. 
The formatter is supposed to receive the input as argument, rather than read from the current Scribe. You can put the HTML into a dummy DIV container if you want to do DOM operations on it.

(Ideally, we'd update the formatter API to receive and return DOM trees rather than strings, so each formatter doesn't need to reparse a new DOM tree, and we could try and retain identity of DOM nodes, for #209)