Add nodes a 'color' property.

Add nodes a new property as 'color'.
Render the label of a node by a specified color if that property is set. 

For example:
```js
var michael = graph.newNode({label: 'Michael',color: '#00A0B0'});
var jessica = graph.newNode({label: 'Jessica', color: '#6A4A3C'});
var timothy = graph.newNode({label: 'Timothy', color: '#CC333F'});
var barbara = graph.newNode({label: 'Barbara', color: '#EB6841'});
var franklin = graph.newNode({label: 'Franklin', color: '#EDC951'});
var monty = graph.newNode({label: 'Monty', color: '#7DBE3C'});
var james = graph.newNode({label: 'James', color: '#00A0B0'});
var bianca = graph.newNode({label: 'Bianca', color: '#6A4A3C'});

```

![2014-10-30 18 59 15](https://cloud.githubusercontent.com/assets/6156889/4841526/8057a26a-601b-11e4-91c5-30c818b058d3.png)