Fix for Issue 10 in ep_tables

Fix for Issue #10

In some cases (like when looking at the time slider) it turns out what etherpad says is a node, actually is just a normal javascript object with innerhtml defined.
Trying to call DOM functions on this node will reveal it for the phony that it is, and it will throw an exception.

This fix disables the tables functionality in the timeslider perspective, but at least it's still usable.
I couldn't find an elegant cross-browser solution to checking if an object was really a DOM node or not, so I just check to see if it has a tagName. DOM nodes do, and fake dictionaries don't .

We should start a discussion over at etherpad,  they should update their documentation with a warning that acePostWriteDomLineHTML might not always be given an actual DOM node.