Added method of intersection for ImageSet with Integer as baseset

I'm not sure if this ok.

`n` and `m` - dummy variables, they shouldn't affect the result.

```
In [1]: hash(Lambda(n, n))
Out[1]: -2984828439234142848

In [2]: hash(Lambda(m, m))
Out[2]: -2984828439234142848

In [3]: hash(Lambda(m, -m)) # but!
Out[3]: 6063053752394237392
```