Replace Map<String,String[]> with serializable Metadata

Having a wrapper for Metadata would be a bit more elegant than moving the Map<String,String[]> around.
This initial implementation adds Metadata and removes the KeyValues utility. Later on we can replace all occurrences of Map<String,String[]> and systematically use Metadata instead.
we don't. The formatting rules live in [https://github.com/DigitalPebble/storm-crawler/blob/master/eclipse-formatting-profile.xml]