- changed class visibility of  header view objects
- changed visibility of mHeaderThresholdHeight field
- introduced mHeaderView2 field
- extracted method for refresh timer setup
- changed setPullToRefreshHeaderView to add and return PullToRefreshHeaderView (for caching purposes)
- added onBeforeEndRefreshing callback
- reworked setHeaderHeightInternal to include new callbacks (onEndPulling and onEndRefreshing)
- interface changes (include new callbacks)
- PullToRefreshHeaderView extended with getters and initialSetup (called on first lay out)