Add missing "uuid." prefix to gml:id

AIXM Feature Identification and Reference 1.0, page 7:

| It is recommended that the gml:id of the AIXM features (such as
| aixm:Airspace, aixm:Runway, etc.) also make use of the UUID value,
| prefixed with "uuid.".

http://www.aixm.aero/gallery/content/public/AIXM51/AIXM_Feature_Identification_and_Reference-1.0.pdf#page=7
Merge pull request #7 from m-click/add-uuid-prefix

Add missing "uuid." prefix to gml:id