add method

I add method setIndicatorLength(float). this method will shrink the indicator length in exactly the length we set. indicator will be center under the tab.

another method setIndicatorLengthScaleOfTab(float Ratio) ratio is from 0 to 1. make the indicator length  ratio * tabLength 

others:
In field ,three variable was added. they are IndicatorLength  , indicatorRatio and indicatorCalculatorState.