`piecewise` exposes raw memory

[stefan]

piecewise uses empty to allocate output memory. If the conditions do not sufficiently cover the output, then raw memory is returned, e.g.,

import numpy as np
np.piecewise([0,1,2],[True,False,False],[1])

A patch which addresses the issue is available here for review:

http://codereview.appspot.com/1105

Documentation is being updated on the wiki.

[stefan]

Applied in r5360.