Added guideline-compliant .icns file (for OSX)

I used the current icon (well, a more high-res version of it) to create a proper .icns file to use as an icon for OSX. I've added some shading that makes it more compliant with the general look and feel of Dock icons.

I was not sure where to put it, as the repository did not seem to contain the currently-used .icns file, so I ended up placing it in /src/Resources.