adding export_responses_by_attr function to remotecontrol

Hi,

I needed to filter and export survey responses based on certain values, and so I came up with this addition to the remote control.

I'm new to LimeSurvey and Yii (as well as GitHub), so there were some things I was confused about and should probably be changed... for example I would have preferred to leave the $oFomattingOptions->responseMinRecord and MaxRecords blank and just get everything back, but I had to set them to get more than one record.

The comments are just there for my own edification as I was figuring out what was going on.

Thanks!  Hope this helps someone.