Transformer convenience

This PR simplifies the creation of classpath and project transformers by creating a ```transformNode``` convenience method that takes a parent name and a function ```Seq[Node] => Seq[Node]``` for transforming a set of child nodes to a new set of child nodes.
There are some predefined implementations of such functions but the user can also easily use his own.
A usage example can be seen in sbteclipse-plugin/src/sbt-test/sbteclipse/02-contents/project/Build.scala

Best
  Martin