Add setter for the ID_ATTRIBUTE_NAME to top-level React

[dvdplm]

To facilitate use-cases where multiple React apps are running on the same page,
expose the DOMPropery ID_ATTRIBUTE_NAME on the top-level React object and make
ReactMount reference the propery from DOMPropery rather than its own local copy.

This branch is based on 0.13-stable. Issue with background: #1939

[zpao]

We definitely won't do this for 0.13 so I'm just going to close out so we can focus on the other PR.

[dvdplm]

@zpao that makes total sense (and I don't expect the other one to be merged as-is either, but one has to start somewhere right?). :)

[hakunin]

Whats the best way to set DOMProperty.ID_ATTRIBUTE_NAME these days? I tried setting it at the top if my code, but then get errors with bindings.

[hakunin]

Lot of people say they have to fork React just so they can modify ID_ATTRIBUTE_NAME.
While its great that v15 doesn't need this, it would be best if we could avoid the forks and just be able to set this variable in v14 now.