TypeError: Cannot read property 'feature' of undefined and some other minor fixes

* Fixed annoying 'TypeError: Cannot read property 'feature' of undefined' error.
* Extended Leaflet Slider the leaflet way
* use [] instead of new Array() .... http://www.yuiblog.com/blog/2006/11/13/javascript-we-hardly-new-ya/
* Updated index.html demo
* Updated readme file
* Tabs vs Spaces

:+1: