#8 Support for GUID/ID

[Wim Bokkers]

This is needed by RSS popper.

RSS Popper holds an ID for every item seen so far:
If there's a GUID/ID field (see RSS/RDF spec) - use
this field, otherwise use the 'Link' field. If RSS
Popper finds a new ID, it assumes it's a new item, and
create a new mail item.