Fix Issue 2021: cross product always returns (1,3)-Matrix

TODO add docstring like in dot and check `len(a) == len(b) == 3`

This is the patch from [issue 2021](http://code.google.com/p/sympy/issues/detail?id=2021).  I would have just pushed it, except I don't know how I feel about it, so I want other people's opinions about the change.