Fixes #5 - Added a tutorial menu.
Merge commit '62958ca' into 5-tutorial-2
Merge pull request #19 from AdrianGaudebert/5-tutorial-2

Fixes #5 - Added a tutorial menu. 
Close #5
[Feature #5] Fixed: Pad tutorial was never shown.
Fullscreen disabled, because it caused problem with anti-viruses when executed from the .exe.
Resolution is now 90% of user resolution, it's better because the window fits the screen.
[Fix] dt was not used in certain game update calls, thus leading to very fast (or slow) movements on certain computers.
Missiles are faster.
[Balance] Difficulty balanced.
Missile upgrade now also increases the speed.
Some sounds are rewinded so they can be played quickly multiple times without any sound bug.
[Pad controller] B or X in upgrade mode cancels the action and in pause or upgrade menu, closes the menu.
[Balance] Tried to balance the game a bit.