Added Touch support, coordinate system, gridlines

- Added touch support to GKLineGraph (see demo for an example)
- Added delegate to detect touches in GKLineGraph
- Added support to configure the line-dashpattern of the gridlines in GKLineGraph
- Added support to display x- and y-axis in GKLineGraph
- Added support to display gridlines in GKLineGraph
- Added support to configure the color of the axes and labels in GKLineGraph
- Added support to configure the color of the gridlines in GKLineGraph