Added intro text to the start script.

My sister was overwhelmed by all of the tutorials and the complexity
of the game, so I've added a short introduction to the start of the
game, to give you a quick introduction, enough to get you going
without being too heavy.
End the start event all the time.

I made a mistake and got rid of the ending of the start event in
one part of an if statement. This has been fixed.
Added some text to the intro.
Simplified the start text.
Modified the intro to conform to the new startup text.

The intro I added, showing you controls and such, is set at right
after a flight exam and explicitly says that the ship you pilot is
a gift from your instructor, so the line about how you take a ship
and fly off is conflicting now. It doesn't really add anything, so
I just removed it.
Merge branch 'simpleintro' of https://github.com/onpon4/naev into tmp