process imaginaries to determine whether expr is positive or negative

pairs of imaginary factors change the sign of an expression; detecting these was added to Mul._evalf_is_negative.
> I*sign(t) is positive or negative

Tests are already in place that show this is not the case:

```
>>> from sympy import *
>>> t = var('nn', nonnegative=True)*I
>>> q=I*sign(t)
>>> q
-sign(nn)
>>> q.is_positive
>>> q.is_negative
>>> q.is_nonpositive
>>> q.is_nonnegative
```