make root & relativeTo options relative to source tree

Previously the options would be relative to the current working directory, which didn't really make any sense.

I also used the opportunity to only create one instance of CleanCSS.
I think we need not to add hook here. We should do the same thing [when initializing `CleanCSSFilter`](https://github.com/shinnn/broccoli-clean-css/pull/1/files#diff-168726dbe96b3ce427e7fedce31bb0bcL6) instead.

What do you think?