Refactor MotionMark code

[shallawa]

The benchmark workflow and the score calculation are very tightly coupled. We need to make the calculation of the score separate such that experimenting and hacking it is easier.

The regression calculation has some bugs. We need to fix these bugs and we need to follow the algorithm of the two segments regression since its correctness is proved.

The variance of the samples is filtered in many ways but still the end result is not very satisfying for all the browsers. We need to remove the cause of the variance rather than filtering unwanted samples.