Exceptions now go into CV_Error. Added thresholding to the FaceRecognizer and updated the demo accordingly.

Several exceptions added to the available FaceRecognizer classes and helper methods, so wrong input data is reported to the user. facerec_demo.cpp updated to latest cv::Algorithm changes and commented.

always remove libopencv_world.a before building framework to always include the up-to-date code into the framework binary.

added script to build the universal opencv2.framework for iOS device and simulator. fixed opencv_world cmake script to create static libopencv_world.a for iOS. temporarily disable compressed file storages in the case of iOS (to avoid dependency of zlib)

possibly fixed crash boost caused by the recent modification; slightly improved mlp params in letter_recog sample