Implement DB Metadata, Group UUID

[DavidVentura]

This fixes #42 This commit implements a Metadata field for Database, whose only member is 'recyclebin_uuid' (String).

It also implements a uuid field for Group, that can be matched with the database's meta.recyclebin_uuid

I'll try to figure out how to get this tested as well. I don't know anything about the KDBX formats so I'm not sure if this also applies to v1 (which seems to not share the parser compared to v3/4?)