Offset is not updated when slicing memmaps

<pre>
>>> import numpy as np
>>> a = np.ones((10, 10))
>>> np.save('a.npy', a)
>>> b = np.load('a.npy', mmap_mode='r')
>>> c = b[3:]
>>> c.offset
80
>>> b.offset
80
</pre>

The offset of c is wrong: it does not take in account the fact that the slicing has changed the offset.
