Numpy, grouping every N continuous element?

I would like to extract groups of every N continuous elements from an array. For a numpy array like this:

a = numpy.array([1,2,3,4,5,6,7,8])

I wish to have (N=5):

array([[1,2,3,4,5],       [2,3,4,5,6],       [3,4,5,6,7],       [4,5,6,7,8]])

so that I can run further functions such as average and sum. How do I produce such an array?