Removing jQuery dependency

While leaflet is framework independent, the dvf plugin requires jQuery which I believe is not needed.

This pull request removes the dependency, replacing the use of jquery with tools from the leaflet library like L.DomUtil.create, L.DomUtil.addClass and standard cross-browser javascript (createElement, innerHTML, cssText...). The code is similar in size and clarity.

A few functions were expecting or returning jquery objects. They now expect or return standard DOM element instead.

Examples have been updated.