Hg remove empty ps

This fixed the problem of <b> tags wrapping <p> tags and creating invalid HTML when pasting from Google Docs. 
"as the formatter will fix invalid HTML by unwrapping child nodes (e.g. the <p>s inside <b>)"

It's worth nothing I think this formatter (fixing invalid HTML) should probably be separated from scribe-plugin-sanitizer at some point, and moved into core.