Added features and modifications

While using this library, I found some parts that weren't done or required cleaning.
1- The legend now displays down vertically
2- The DoughnutChart can now detect touch using the new DoughnutMapper class that extends PieMapper
3- Users can set whether they want values to be displayed at the center of the doughnut
4- There is a tiny margin now between each slice of the doughnut chart
Example: 
![example](https://f.cloud.github.com/assets/3849459/1695925/9c4138aa-5eba-11e3-9850-ee237d6f6344.png)

Issue: When the value is 0, there is a small spacing that shouldn't be there.