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