support functors

[aminroosta]

Trying to fix #11 :-)

[Killili]

The new test compiles and runs in VC2015.
I tried to get an understanding of the fault but i'm lost in the template jungle... i have the suspicion that there is a easier way but i might be totally wrong here. Shouldn't std::function in itself take care of the lambda/function/functor/member stuff?

But on the other hand this works, so you could pull this.

[aminroosta]

@Killili Thank you for testing on VS 👍
I was looking at function_traits the other day, and i saw that it specializes on operator() const member function, but user defined functors usually just have operator() without the const keywoard.

I tried specializing it without the const keyword and compiler seems to be picking the write specialization in case of both lambdas and functions.