adds airspace data, openair format parser.

this commits adds airspace support, with appropriate objects defined in
the common package - new structs Airspace and AirspaceSegment.

it also adds support for the OpenAir format, as defined at:
http://www.winpilot.com/UsersGuide/UserAirspace.asp

this is the first plugin implementation for airspace data.

Fixes #24.