include metadata in manifest similar to sbt package

[karchie]

sbt package leaves useful metadata in the project jar (Implementation-Vendor, Implementation-Title, and especially Implementation-Version). none of this is present in the jar built by the webstart task. it would be really nice to have these (mostly to support package.getImplementationVersion)

[ritschwumm]

i could probably just use whatever the package task packages instead of creating my own jar - but, to be honest, i have no idea how to do that.