relative.image.dir should be relative to dir.source

The relative.image.dir property was created relative to the
project base dir, which means that, when dir.source != ., the name
of the source directory was included twice.

Add the basedir parameter to determine the relative path from
dir.source.
Merge pull request #160 from gmacon/relative-image-dir-source-not-cwd

relative.image.dir should be relative to dir.source