Added In-place resource optimization

hello,
  I added the ipro related code as apache instaweb_handler. but it still can not pass the `In-place resource optimization` test. 
 it seems that `/ipro/test_image_dont_reuse.png` could not be rewrite in-place. in ```ps_inplace_checker``` the response status code is 404. Did i miss anything important? maybe you can fix `In-place resource optimization` based on this pull request. the key patch is https://github.com/pagespeed/ngx_pagespeed/commit/d883d5810d751b6722cbdcdf052170fc39c88979, others are just preparation.