Add variable angular speed + refactor.

Angular speed of the ship is function of its current speed.  That is,
        the slowe it travels, the faster it can turn.  It still turn
        fast enough to let the ship steer.

Refactor some errors (vertice -> vertex), some variables that should
have been local, and transform a loop with manual iterator construct to
use the foreach construct instead.
Merge pull request #5 from aybabtme/master

Ship's turn rate function of its speed.  Also little refactor.