Change BinaryReader.readList API and add Commit.parentIds

The API of the method net.secloud.core.utils.BinaryReader.readList has
been changed to be more consistent to its writeList pendant.

readList[T](inner: => T): List[T] --> readList[T]()(inner: => T): List[T]

net.secloud.core.Commit can now store references to its parents.