Add Rectangle.ZERO field
Use some iterators in FlightPathGenerator
Rework airspace to make it easier to use with network code

This commit has 2 main changes (other changes are to ensure things still
compile).
- Landing causes planes to be culled and taking off generates new aircraft
  objects. Before the Aircraft object itself was stored in a list.
- Flight path generation is initiated from the airspace class (instead
  of within AirspaceObjectFactory).
  + This avoids various hacks needed before to force an aircraft to accept
    a new flight plan.
  + It also means that there is no element of chance involved with the
    AirspaceObjectFactory. This is needed for the network code since the
    client will need to FORCE the gui to create a plane when the server
    tells it to. This was not possible to do (without massive hacks) before
    this change.
Changes to getBearing/Angle fixes #30
Remove code I forgot to big airspace commit
Logic refactoring + dead code removal (fixes #31)

- Most dead code as a result of the previous update
- Changed newer methods in AirspaceObject to be optional
Removed calculateAngle fixes #30

(properly this time)
Merge remote-tracking branch 'github/master'
Rewritten Airspace takeoff tests so they work properly (fixes #32)
Update to libGDX 1.0.0

Changes required
- Drop sonatype repository (libGDX is now in maven central)
- Stage viewport system has changed
- SpriteBatch class was renamed to just Batch
- cfg.useGL20 is now enabled by default
Use FitViewport in AbstractScreen

Causes black bars to appear on foreign aspect ratios instead of stretching
the game.
Throw exception on empty entry point list fixes #29
Fixed failing
Very minor stuff (me being picky)
Really fix #29
Move the ConcreteAircraft factory within that class
Fix highscores order and add a limit on 10 high scores
Merge pull request #33 from sepr-hou/fit-viewport

Use FitViewport in AbstractScreen.
Options Screen fiddled with
Moved highscores table so title not obscured