1. real gravity support
2. solve bug of zero velocity
3. added multistep function to system
4. better comment
Real first order approximation. (previously the velocity is just mean velocity
over the past dt, now the actual velocity is approximated.

with step dt = 0.05, end point height raise from 1.01 to 1.16
rotate the velocity at each step without changing the magnitude, then
accelerate it at each step, magically, the accuraccy is greatly improved. about
10 times more accurate.

I will not care anout the accuraccy anymore for the time being.