Refactor of ChangeEventPlugin

some refactoring and also handle if `blur` doesn't fire on the form
input in IE8 (by always cleaning up on focus). per discussions with
@balpert
Change ReactID.ATTR_NAME to "data-reactid"

This final change is what we've all been waiting for.

Note that it no longer makes sense to use `document.getElementById` in
`getNode`, because that only ever worked with "id" attributes.