restrict multiplicative extraction of negative sign

https://code.google.com/p/sympy/issues/detail?id=3973


No tests fail when making this change and it will alleviate confusion about what is supposed to happen during extraction: the underlying form of the expr should not change: so the sign of the expression should not change unless a negative is extracted from a negative expression:

```
>>> (4*x).extract_multiplicatively(-2)  # there's no negative
>>> (-4*x).extract_multiplicatively(-2)
2*x
>>> (-4*x).extract_multiplicatively(2)  # -1*4*x so extract 2 from 4
-2*x
```