Avoid unnecessary array allocations in dangerouslyRenderMarkup.
Supporting mounting into iframes

Sencha says that separating big components into their own iframes was important for performance:
http://www.sencha.com/blog/the-making-of-fastbook-an-html5-love-story.

Today the only thing stopping us is that events don't bubble to our events system from an iframe. This diff
looks at the owning document of the container and adds top-level listeners to it. It should not change
existing behavior and should improve our support for this.
Log unknown props only when we have a match

Logging every unknown property got very noisy when combined with use of `transferPropsTo`. I knew this would be a potential issue initially but decided it was worth it. Others disagreed and it's resulting in some confusion.

This changes the logging to ensure that we have a potential correction, so only DOMish properties should result in warnings.
Stop using comments as boundary markers in dangerouslyRenderMarkup.

Jordan warned (and StackOverflow confirmed) that IE8 doesn't respect HTML
comment nodes when setting `.innerHTML`:
http://stackoverflow.com/questions/15006001/inserting-a-comment-in-innerhtml

The virtue of the comment strategy was that the parser did all the work of
maintaining the boundaries between markup chunks, using comments as the
boundary markers.  The new strategy (of tagging the first node with a
special attribute) has a similar virtue, since the parser should preserve
that attribute only for the nodes we care about, and any rendered nodes
that do not have the attribute can be ignored (and complained about by
`console.error`).