Provide generic Interpolator

[huan-nguyen]

First of all, nice library and great animation. However, I think the interpolators at the moment is created based on some pre-calculated constants. For example, in ActionBarInterpolator:

   private static final float FIRST_BOUNCE_PART = 0.375f;
   private static final float SECOND_BOUNCE_PART = 0.625f;
   @Override
   public float getInterpolation(float t) {
       if (t < FIRST_BOUNCE_PART) {
           return (-28.4444f) * t * t + 10.66667f * t;
       } else if (t < SECOND_BOUNCE_PART) {
           return (21.33312f) * t * t - 21.33312f * t + 4.999950f;
       } else {
           return (-9.481481f) * t * t + 15.40741f * t - 5.925926f;
       }
   }

The constants used in getInterpolation (e.g., -28.4444f) are pre-calculated based on the values of FIRST_BOUNCE_PART and SECOND_BOUNCE_PART. If either or both of these values are changed, the constant values need to be recalculated.

This limit users of this library from changing the value of FIRST_BOUNCE_PART and/or SECOND_BOUNCE_PART to whatever they want because if they change these values, they have to do their own math to work out formulas for the interpolators.

So I think it would be nicer to provide generic formulas for interpolators with perhaps FIRST_BOUNCE_PART and SECOND_BOUNCE_PART are variables to avoid users being worrying about the formulas themselves.

What do you think?