GROOVY-6867: Added init(), dropRight() and takeRight() to Iterable, List and Object[]

Iterable, List and Object[] have tail(), drop() and take() but init(), dropRight() and takeRight() are missing.
I added them.

Method names are from Scala.
http://www.scala-lang.org/api/2.10.4/index.html#scala.collection.immutable.List