Fix for no SpriteMultiple width/height copying

The topheight, rightwidth, bottomheight, leftwidth measurements weren't
being copied over so Springboard didn't know what to do with them.
Added spriteMiddle stretching to SpriteMultiples

If the sprite in sprites.js specifies middleStretch to be true, the
inside image is used as an image, not a pattern.

Fixes #62.