autoplay runs when attribute not present

Fixed an bug in the autoplay plugin where autoplay functionality will fire in the case where the `data-autoplay` attribute is not on the element, the `attr("data-autoplay")` function will return `undefined` and the conditional will evalute `true` because it only tests for not null and not false. 

The fix will will permit autoplay to fire in cases where the attribute is empty '', or set to anything except for 'false' (case-insensitive).