feature/static labels

Feature to add static labels on top of bars.
Allowing callback for content of label:
![morris_static_label_callback](https://f.cloud.github.com/assets/1506905/1369097/58b097de-39d5-11e3-9274-6c7a9a3c46ca.png)
```
...
        labels: ['Distance of'],
        staticLabels: {
            margin: 20,
            labelCallback: function(value, options, label) {
                console.log(arguments)
                return label + ' km'
            }
        },
...
```
![morris_no_static_label_callback](https://f.cloud.github.com/assets/1506905/1369098/597967f4-39d5-11e3-8dcf-2893156b0207.png)
```
...
  labels: ['Y', 'Z'],
  staticLabels: {
    margin: 20
  },
...
```