#169 Copying sources from core-common to core/core-jdk8 using maven-resources-plugin

... to allow faster builds and nicer roundtrips in IDEs.

I noticed that ```mvn clean test``` doesn't work as expected, as core and core-jdk8 require a generated source-jar in core-common. Plus, when I changed someting in core-common in the IDE, I always had to perfrom a build on the console for the changes to show up in the IDE.

This PR fixes that: m2e understands nicely what the resources plugin wants to do (at least during a clean :wink:) and on the console I can do ```mvn clean compile``` :)