Work Progress - Oct 27 - Nov 2

Work done:

* Change SB background to a darker one, so it is not that distracting.
* Attempt to speed up simulation:
-- Increasing the step size [adjustable as an advanced property "Speedup factor" of the BoarderEngine object]: It produces less precise collision simulation.
-- Changing the gravitational constants [adjustable as basic properties "X-gravity" and "Y-gravity" of the BoarderEngine (background) object]: It affects the simulation result.
* Negative acceleration done: Try triggering Accelerate() when the skateboarder is moving downhill.
* Implement brake() which does not accelerate when the speed has been reduced to zero. (Remarks: Negative jump doesn't make much sense as now jump is now defined as providing acceleration towards the direction of the surface normal (i.e. away from the surface). Negative jump then means accelerating towards the surface, which means no effect at all after immediate collision. Also, it doesn't seem a skateboarder can exert a force that is directed towards the ground.)
* Patterned placement of line segments: try adding line segment/broadline2 objects
* Rename RocketKick() to JetpackPush()