Duplication in the Elements tree

[rickbrew]

Looks like this actually broke back in #237 .

When we transmit DOM changes to Chrome, we need to keep track of all the elements for each transmission so we don't send over duplicates. When we send over DOM.childNodeInserted(E), which includes E and its sub-tree "by value", we must not send over DOM.childNodeInserted() for any children of E, otherwise they'll be duplicated.