rescale() should accept 2-dimensional scale

[andykee]

There's no reason we can't rescale differently in row vs. col (or x vs. y)

I think the change is as simple as sanitizing scale to accept a single or multiple values, and updating the following block to use the correct indices:

if shape is None:
        shape = np.ceil((img.shape[0]*scale, img.shape[1]*scale)).astype(int)
    else:
        if np.isscalar(shape):
            shape = np.ceil((shape*scale, shape*scale)).astype(int)
        else:
            shape = np.ceil((shape[0]*scale, shape[1]*scale)).astype(int)

    x = (np.arange(shape[1], dtype=np.float64) - shape[1]/2.)/scale + img.shape[1]/2.
    y = (np.arange(shape[0], dtype=np.float64) - shape[0]/2.)/scale + img.shape[0]/2.