Add icon to oracle

A typical problem is that users don't know when/why/how they need to run oracle. By now it's also scary: it doesn't provide an icon, so users are not even sure if they are supposed to run it.
This commit adds an icon for the three major oses (windows's .ico, osx's .icns, linux's svg+png).
The icon depicts the back of a card with a superimposed "O".
I am not a designer and i would welcome someone providing a nicer icon.