Cross compile for scala 2.9.3 and 2.10.1

This follows the sbt convention of adding the version number to the artifactId separated by an underscore. I think it's better to do it this way rather than using classifiers, because this way is fairly clean, and doing it with classifiers would make it hard to add as a dependency to an sbt project.
This is the way it's done in the scalatest project: https://github.com/scalatest/scalatest/blob/master/pom_template.xml

This may be more of a starting point, because I don't know what changes should be made to the parent pom (and to the scalagen-maven-plugin pom). Is there a way to do it without duplication? I'm not sure.