ViewPager with pageMargin over-scroll not work when scrolling forward and backward

[tamhuynhit]

I face the same issue as the one mentioned in #35 and #23

The Over scroll effect does not work properly when I use a ViewPager with setPageMargin to a non-zero value. I found out the problem comes from the ViewPager because it doesn't give us the correct positionOffset in onPageScrolled. The offset is very small nearly to 0 but not 0, the same for 1.

So a way to fix this is to compare the float mLastPagerScrollOffset approximately and it works

Please test this and let me know if there is any issue