fixed ds-graphscan test to make sure that the memory block size is large enough to fit at least the graph header and the sequence block header (ticket 504)

fixed test failure in matrix-dotproduct test (by increasing the desired accuracy threshold) (ticket #447)

refactored gpu module.

the path to "opencv/opencv_extra/testdata" can now be set using OPENCV_TEST_DATA_PATH. Filter pattern can now start with ">" or ">=" symbols, meaning - run all the test, beginning from the particular one - convenient for resuming tests after fixing crashed tests

added Mat::push_back, pop_back and related operations; enabled reading/writing/creating/copying matrices with zero dimensions.