Fixed: next can only be pressed once and not while scrolling; fixes #48

[aforty]

Issue #48 describes how the slides would get "stuck" when the next button was pressed too quickly in succession. This was because it would attempt to slide "next" while in a half-state.

This fixes the issue by keeping track of the scrolling position internally as opposed to relying on Swiper's internal isScrolling state which appears often to be wrong or lagging.

[aforty]

Any movement on this? Not having my app be stuck would be helpful.