Add Positive assumption helper for gamma function, Fixes #8524 

The Positive assumption helper for gamma function is missing:
So the the following code returns None:

```python
>>> n = Symbol('n', real=True, positive=True)
>>> gamma(n).is_positive
```
With this PR, Positive assumption works well for gamma function.

Fixes #8524 
I am unable to understand what exactly this means, can please clear with an example?
>You could take numbers.  I.e. unevaluated gamma function for rational
argument.