Initialize first/last VisibleItem to avoid NaN cascade

Fixes #16

_firstVisibleItem is never initialized which results in percentInView,
this.scrollTop and newFirstVislble being calculated as NaN.

Initializing both _first and _last VisibleItem values to zero fixes the
NaN issue and cleans up the rendering issue when initializing LiteList
with an out of DOM element.
Merge pull request #18 from davidcl64/issue_16

Initialize first/last VisibleItem to avoid NaN cascade