Expose getter functions for min, max, stiffness, dampingRatio & elasticBehavior.

Add option to set the default inside color of the thumb either programmatically (setDefaultThumbInsideColor()) or in XML (app:defaultThumbInsideColor).

Fix setCurrentStartValue (& setCurrentEndValue) bug in RubberRangePicker.
Add attributes to set initial values for RubberSeekBar/RubberRangePicker directly from XML.

Min supported android version is now API 15.

Fixed incorrect values being returned by RubberSeekBar(getCurrentValue) and RubberRangePicker(getCurrentStartValue & getCurrentEndValue).
Fixed incorrect values being set by RubberSeekBar(setCurrentValue) and RubberRangePicker(setCurrentStartValue & setCurrentEndValue).