added Framework target and configurable highlight line on touch

[dhf]

• Add a framework target to use swift-linechart easily as submodule in other Xcode Projects
• Add the possibility to display a highlight line on touch additional to the highlighted datapoints. The following properties ca be configured for the highlight line:
• visible true/false (default true)
• color (default UIColor.grayColor())
• lineWidth (default 0.5)