v3.0.0

1. rewrite all iOS codes， improve scroll performance/Smoothness, fix scale/onPageChanged...
2. add onPageSigleTap(), onScaleChanged()
3. update android dependent lib AndroidPdfViewer to 3.0.0.alpha.5
4. android support link in pdf
5. delete cache file when error happened

Known issues
iOS zooming can not scroll to pinch center