This allows subclasses of CvBoost to override initial weights that
update_weights uses without duplicating its entire implementation.

(Sometimes sample_data became bad because the buffer with its values was deallocated early).

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

fixed potential buffer size overflow in Boost::update_weight, bug #1524 (thanks to benlemna for the patch)

replaced alloca() (a.k.a. cvStackAlloc) with AutoBuffer or vector() everywhere. cvStackAlloc() is still defined, but we do not need alloca() anymore to compile and run OpenCV (fixes #889 and may be some others)

added more helper macros to the function declarations, to assist the Python wrapper generator. Fixed memleak in Mat::operator()(Range,Range) and the related functions (Mat::row, Mat::col etc.)