Add meta-information for dbuild.

The next version of [dbuild](http://typesafehub.github.io/distributed-build/0.5.3/index.html)
will parse `dbuild-meta.json` to determine which jars are produced by a Scala build.

This way we can modularize without changing dbuild itself.

Yes, I know `dbuild-meta.json` should be generated during the build.
However, given the state of our build.xml, I think this is pointless.

My goal is to generate build.xml, dbuild-meta.json and Eclipse projects
from a higher-level description of our build. Baby steps...

Including improvements by @cunei:
  - Removing outdated field "uri" from ExtractedBuildMeta
  - Changed "partest" to "scala-partest" (the actual jar name)