Move license-maven-plugin config off the server

Previously, we stored the license configuration at:

    http://code.imagej.net/licenses

But it was a pain to keep that updated and in sync with the codebase
here. It makes more sense to use a local license resolver.

Thanks to Johannes Schindelin for the idea.
Add project.rootdir that always points to toplevel

This is a bit of a burden to maintain when projects are added or moved,
but makes it easier to point to files off the root path. Usually doing
so is a bad idea anyway, but in some cases it is nice to have access to
shared configuration, such as the files in the license folder.