Added support for scrolling with a GraphView placed in UIScrollView

To enable scrolling with a touch that begins on a GraphView I made two changes, first I exposed panView and panGesture as properties. Second I implemented the gestureRecognizerShouldBegin method, and had it dodge panning in the vertical direction, instead sending it to the parent view which means that scrolling a UIScrollView will work as normal.

— Nils
Merge branch 'feature' of https://github.com/nmattisson/BEMSimpleLineGraph into feature