Calling several times in the same request to start/sendData fails

Hi,

When you want to make profiling in separate files, you need to call XHProfiler::start() and XHProfiler::sendData() more than one time.

But the XHProfiler::$started not is restored to default when XHProfiler::sendData() is called so XHProfiler::start() dont make nothing the second time returning false. 

Regards and thanks for your job.