Add copy / rename option

For me, grunt-contrib-copy corrupts images. In finding a workaround, I modified this task to have a 'copy' option that saves the image without resizing, as well as allowing renaming. In total, it involved adding 2 lines and changing 2 others.

Do you think these changes belong in this task, or should there be a separate image-copy task until copy and concat work out their image problems?
