Dot product overwrite bugfix

This is a proposal for fixing issue https://github.com/numpy/numpy/issues/3357. It goes beyond simply warning the user about this, which was the least we want do based on the ML discussion:
http://thread.gmane.org/gmane.comp.python.numeric.general/54277

This makes a copy of the entire array that is overlapping to do the "right" thing, assuming that no-one would willingly want to do the dot product between matrices that are changing during the dot product.

Feedback requested.