Don't output hsl / hsla colors, for browser compatibility

The original sass converts hsl / hsla colors into rgb, and usually outputs hex colors, so the result works in old IEs. According to caniuse, IE7 and IE8 don't support hsl / hsla.

This patch guts `__str__` in ColorValue to obtain the desired behavior. I hope you'll consider merging it, and I'm willing to implement the patch in another way if you don't like this approach.

Thanks!