Patch 0.1.4

- Added Difficulty Selection.

• There are four difficulties : EASY, NORMAL, HARD, ACE.
• Difficulty affects the following properties:
  • Player's aircraft durability (*in ACE difficulty, you will be shot down with just one missile.)
  • AI Evasion rate and evading performance
  • Missile performance launched by AI

- Fixed an issue: When you fail at phase 3 and shot down Pixy at the same time, "Restart from checkpoint" didn't work properly.