First open source (demo) release, a.k.a. "Learning to fly"

This release should serve as the first presentation of the open source Jetpaca to the world. It should have (non exhaustive list):

• Full compatibility with Godot 2.0+
• Refactored code to be more modern, use the Kobuge style guide and be ready for external contributions
• A small set of well-polished levels, showcasing the main gameplay elements (and with no visible bug)
• A good looking UI

Future releases would then focus on improving the gameplay, add a lot more levels, possibly some more art and enemies, music, and a storyline.