Fix: `options.annotation` should be relative to `options.to`

If `options.annotation` is set to a path, that string is inserted
after "sourceMappingURL=". That means that the path will be relative to
the output CSS file -- in other words, relative to `options.to`.
Merge pull request #54 from lydell/annotation-relative

Fix: `options.annotation` should be relative to `options.to`
Better prev source map support docs
Fix relative paths from previous source maps

Before, all previous source maps were assumed to be located inside or
next its corresponding file. However, it could be saved in a different
directory. If so, all relative source paths in the previous source maps
must be rewritten according to _that_ directory.
Merge pull request #55 from lydell/sources-relative-to

Fix relative paths from previous source maps
Decrease properties length
Add test for PreviousMap#root