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.

Fixes #31.