Fixes #8488

(This is my first PR :D )
Implemented _eval_is_prime for the Mul class in core module

Example:
k = Symbol('k', integer=True)
(9*k).is_prime
False
On Mon, Nov 24, 2014 at 11:28:15AM -0800, Lukas Zorich wrote:
>    Ohh, sorry, I didn't knew that. Can you give me a hint on how can I
>    determine if something like this "Mul(4, S.Half, evaluate=False)" is prime?

I don't see an easy solution, sorry.  You could take look on other
assumption handlers, e.g. for is_even/is_odd.

This is not an "easy to fix" issue.  If you are looking for such
issues - please take look on labeled "Easy to Fix".

>    The only way I think is to evaluate it first (and I can't use evalf
>    either right?)

No, you shoulnd't use evalf too.  If you want evaluate some object
foo, you can use foo.func(*foo.args).