minpy doesn't support astype function！！！ #185

It maybe change item dtype of ndarray while producing data. for example, if producing image, input is dtype=int, and array need be updated by some aglothims several times and then array are dtype = float32, after that we need to save image back to disk, so we need array of image with dtype=int AGAIN! but minpy DOESN'T SUPPORT 'ASTYPE‘ function which REAL NUMPY support， it really basic， please add them， otherwise minpy CAN NOT BE EASY USE！
thanks a lot!