Added extra block elements to contentcollector.js

Not sure whether this is the correct way to resolve this issue, but there is a bug with ep_headings which means on import, all styles are stripped out from the pad:

https://github.com/fourplusone/etherpad-plugins/issues/38

I am assuming there is a more **proper** way to actually implement this within the actual plugin, rather than the core, but I am at a loss.  If someone could point me in the right direction that would be awesome.