Failing test for memory leak issue #954

renderComponentToString registers events. If the calling code
does not unmount the component or unregister the listeners
this will leak memory. Passing this test should fix this.
Flag to prevent registration of events #954

Flag on EventPluginHub to disable event registration.
For the purposes of server side rendering not registering events.
Could be cleaner if used a transaction with an optional step.
Alternatively could be an ExecutionEnvironment variable.
Could also be flag passed to mount component, however non DOM
components have no comprehension of events.
Test coverage for #954 to allow disabling of event registration

EventPluginHub.setRegistrationEnabled(boolean) will toggle
whether putListener will register event listeners.