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

I updated the network/node image code to accept a broken image url to use when a node image fails to load.  The broke image url to use can be specified via a _brokenImage_ property on a node.

```javascript
var nodeProperties = {
     ...
     shape: 'image',
     image: 'some url',
     brokenImage: 'fallback url to use'
}
```

Let me know what you think.  Basically this stemmed out of viewing networks in social media where the node image being used is a person's thumbnail.  Occasionally these thumbnail images fail to load.

I didn't rebuild the files in the _dist_ directory.  Let me know if you want me to do that.  Thanks!