Added special copy method for usfxr

usfxr lets you read in a CSV string containing all the parameters (which apparently originated with as3sfxr) into a Unity3D app, where you can play them on the fly, tweak them, etc. I made an alternate Copy menu item (opt-cmd-C) that copies in the right format for later pasting elsewhere. Also added a README just to point out how to get SDL in case it's not obvious to someone in the future.