gammasimp can be improved for integer variables

For `a1, b1` being `integers`:

```
In [113]: y1
Out[113]:
Gamma(a1)*Gamma(1 - a1)
-----------------------
Gamma(b1)*Gamma(1 - b1)

In [114]: y1.gammasimp()
Out[114]:
Gamma(a1)*Gamma(1 - a1)
-----------------------
Gamma(b1)*Gamma(1 - b1)
```

Ideally, this should be `(-1)**(a1 - b1)` rather being unevaluated. 

So, I think `gammasimp` can be improved for such cases.