Move to Scala IDE parent pom for plugins.

This allows simpler builds for inclusion in the ecosystem:

* `mvn -DlocalArtifacts=ignore -Peclipse-juno,scala-ide-stable,scala-2.10.x -Dscala.version=2.10.1 -Pset-versions --non-recursive exec:java` to se the strict dependency (against stable release)

* actual build: ` mvn -DlocalArtifacts=ignore -Peclipse-juno,scala-ide-stable,scala-2.10.x -Dscala.version=2.10.1 clean install`