Fix for 2 slide special case

When there are only 2 slides, the actual slides are duplicated in order
to support continuous scrolling. This causes the actual indices passed
to callbacks to be 0..3, when the expected values are 0..1.

This change special-cases the 2 slide scenario and ensures the
externally visible values (callbacks, getPos()), are always within the
expected 0..1 range.

This is essentially a more complete version of the other fixes which
have been offered in other pull requests.
fix slide animation bug. (when duration is set to zero.)
Add optional callback on slide move
Update README
Merge branch 'fix-slide-with-non-transition' of https://github.com/yoonsy/Swipe into fix-slide-with-non-transition
Merge branch 'position' of https://github.com/dpolivy/Swipe into positio