MeanStdDev for RGB images, correct getBoundingBox.

Added MeanStdDev function, which outputs Scalar parameters. Removed then
the function MeanStdDev which worked with single channel images.
In this way, multichannel images are correctly handled. The old
functions mean and stddev have been left for backwards compatibility.

Modified getBoundingBox in order to return correct widths and heigths:
before, they were computed like maxX-minX, now as maxX-minX +1. This
allows to deal with single pixel blobs (which have width and height set
to 1) and to obtain their bounding boxes.
Changed test.

Test for new MeanStdDev function.