Added MacBook Pro device

Hey there,

you created an awesome collection of beautiful device. Well done. I had a need for exactly what you created and a little more. In addition to the mobile devices I needed a representation of a desktop device, therefore I created a MacBook Pro (ish) style in the sense of this library. 
In contrast to the existing devices the MacBook doesn't respect the screen dimensions of the original, for an obvious reason: you cannot display a pixel-perfect MacBook on a MacBook. Nonetheless, it may be of use for someone out there.

Cheers
Klaus