Return asset removed by delete method instead of a boolean

The Archive#delete(ArchivePath) operation should return the Asset (or Node) being removed (at the specified ArchivePath) rather than returning boolean. This both avoids having to have an extra call to get the asset before removing it, and it's consistent with the familiar behavior of the JDK Map API.

// Removes the Asset in the archive at the specified Path and returns the removed Asset
Node delete(ArchivePath path);

The only thing I'm not sure about is whether the Asset return type is sufficient for when you delete a directory path.