PieChart: possibilitie to show x-values and x-values in percent

PieChart: possibilitie to show x-values and x-values in percent
![sans titre](https://cloud.githubusercontent.com/assets/5589240/4461007/08ad2b84-48b7-11e4-8016-ed65b828cb41.png)
LineChart : show Y values of a specific LineDataSet
![sans titre2](https://cloud.githubusercontent.com/assets/5589240/4461011/0bb4d3c2-48b7-11e4-9da6-bc0ceca52ac7.png)