Wrapping add array operation, for additive blends. 

Proceeding with the notion that 1-to-1 match with C API is most ideal, I made 'add' a static member of the OpenCV class so it can be access cv.add(..). I added a new class MatOp, short for math operations, in a effort to organise in a similar fashion the C api as well, do you think it's a good idea to split the functions out like this?