Adding the "cosine_distance" method to the "Ai4r::Data::Proximity" class

Hey guys, 

I've noticed that there was no cosine distance method inside the distance functions module. I'm not sure if you will need this but just in case if you want it. 

I've been using the cosine distance function for my project and it's been working well. I've also created `ai4r-ext` in my repository which I'm planning to use in case I need to continue writing more extensions but I'd like better if I could contribute directly to the main codebase.

Cheers.