fixed broken hole detection in CV contour finder

ofxCvBlob.hole bool now works correctly; updated openCVExample to denote detected holes with some text to make checking this easier in the future; also updated sprintf in example to use a stringstream

This is related to #784 which I thought I fixed, but Chris found I hadn't: http://forum.openframeworks.cc/index.php/topic,11675.msg51514.html