Added Obj-C Tags and Added access to Labels

- I fixed the documentation to use objective-c syntax highlighting so it's easier to read on GitHub
- I added an Array for access to the labels after the graph is rendered so the color can be changed dynamically like the example below

![colorlabels](https://cloud.githubusercontent.com/assets/1218847/3278817/d0017b90-f3d5-11e3-81e0-21bf1c2fdda1.gif)