Attempt smooth scrolling when item is dragged at boundary of list

[fivecar]

Suggested by @karlupo here. Basically, can we smooth-scroll the list when dragging at its boundaries.

In the past, when using LayoutAnimation stuff, it's required UIManager.setLayoutAnimationEnabledExperimental(true) on Android and that sort of thing, as seen here. That sort of setting on Android would be app-wide (i.e. have consequences for the rest of the application), which seems like an over-the-top consequence for integrating draglist into your project. So we almost definitely would need this to be an optional (default false) consequence.