Working prototype

* Loading the plugin causes all click handlers to be wrapped, but doesn't affect click behavior
* Enabling the plugin turns on the wrappers, so click handlers are called only if no second click happens before the specified delay
* Disabling the plugin reverts to default behavior (leaving wrappers in place)

Also includes an example showing the behavior difference with and without the plugin

Note this implementation will only work in Leaflet 0.8 (currently in development). Earlier versions of Leaflet implement event handling in a way that is much more difficult to override.
The state could potentially be associated with (or stored on) the Leaflet map object. Then when running an object's wrapper you'd get its map and find the state. But going from an object to its map appears to require using `_map` which is not a great idea. I'll keep thinking about it.