refactored airfield/waypoint welt2000 plugin.

big refactor of the airfield and waypoint interface definitions, as well
as the welt2000 implementation of this functionality. adds proper
get/put interface, adds the welt2000 plugin config structure, adds mock
implementation of these two interfaces.

also changed welt2000 plugin to have a config value for the release url,
as the rss feed only points to an update page.

also adds the region field to airfield and waypoint structs in common,
and welt2000 now fills them up.

Fixes #31.