add a default value for radius

I added a default value for the radius.  Without this, if the radius is missing, you get Error: Invalid LatLng object: (NaN, NaN) when trying to calculate the bounds for an undefined point (lines 199-203).

The leaflet sample code (http://www.patrick-wied.at/static/heatmapjs/example-heatmap-leaflet.html) also needs to be updated, but I didn't see that in the github source:

radius should have value and absolute fields:

<                     radius: 20,
---
>                     radius: { value: 20, absolute: false },

method to set data has changed:

< heatmapLayer.addData(testData.data);
---
> heatmapLayer.setData(testData.data);