Fixes in minValue and maxValue

minValue and maxValue returned numpy.uint8 objects. This caused problems if you did something like:

    i = Image('lenna')
    mn = i.minValue()
    new = i-mn

The __sub__ function expects an integer or an image. When this passed value is not an integer it treats it as an image and tries to get its bitmap which causes a problem in this case, as it calls getBitmap() on a numpy.uint8 object.
This fixes this problem for minValue and maxValue functions

Similar bug was present in the max() and min() functions also. The functions called getBitmap on numbers, causing errors.