Add nextFrame and prevFrame method

[loudoweb]

There are now a nextFrame and prevFrame methods that allows to move frame by frame all spriters independendly (as long as you update with no elapsed time : update(0) ) There is now a correlation between timeMS and normalizedTime so both values are bounds together. so it allows to change from any time and Spriter calculate the normalizedTime accordingly. There is now a progress property to easily set an animation at a specific progression

Breaking change: change timeMS by time and make normalizedTime read only.

BUG to fix: complete() calls unstable