Tweaked jump physics and endless runner mode

Based on comments from players, this version features reworked jumps physics. The changes can be resumed as such:

• As long as the jump button is held, an upward force is applied. This slows down the fall.
• Increased gravity, so once the jump button is released, the ship falls quickly.
• Increased initial jump velocity to compensate for the increased gravity and provide a similar jumping height.

It used to be that you could only control the magnitude of the jump during the first few frames after pressing the jump button by releasing the button very quickly for a lower/shorter jump. However past those initial few frames, it did not matter if you still held the jump button down or not. The ship would land exactly in the same place. But now, the jump button has an effect as long as it is held down. To me at least, it feels much better. The game also feels easier, not too easy I hope...

Also new in this release, again an idea that was born when discussing the game with players, I have added an endless runner mode! This is simply a new level that never ends and is generated randomly. The ship accelerates until it reaches top speed and never slows down. The distance traveled is displayed in real time, and when the player crashes, game over. How far can you go?

Other changes:

• The ship can now go above the top of the screen instead of just sticking there.
• Miscellaneous level tweaks to accomodate what broke due to the modified
  jump physics and the fact that the ship can go off-screen.
• The PC Booter version now displys Loading StellarRoads... when reading the program from the diskette instead of 1.....2