[Network] Added support for a broken image fallback when node image fails to load

When creating a node, you can now specify a _brokenImage_ property.   This property is a url for an image that should be used when the image specified in the _image_ property fails to load.  This ensures that a node image is always displayed.  This comes in handy when displaying images from other providers (e.g. social media sites) when an image may not always load.

```javascript
// Create node definition
var node = {
     ...
     shape: 'image',
     image: 'some url',
     brokenImage: 'fallback url'
};

// Add node to network ...
```