Add Labels Feature.

Hi everyone,

I add labels for helping users to customize the graph.
The label could be moved.
The link between the label and the point of the graph is adjusted automatically.
Display.java has been modified to optimize the drawing of labels.

This is 2 examples with a radial tree graph.
![crop-mescourbiac22-treemap 1 ](https://f.cloud.github.com/assets/1435052/413324/d2e10b40-abed-11e2-8969-99419de81f03.png)
![crop-mescourbiac22-treemap](https://f.cloud.github.com/assets/1435052/413326/d742bc38-abed-11e2-95a9-5e62af6c8d2a.png)