Sudhakar, Hemanth | Added a config called 'sensitivity' for the point so that click sensitivity can be configured.

Right now, the click area of a point is hard coded in the code and in high pixel density devices (iPad) this is not enough. Have modified the data.js (c3_chart_internal_fn.dist) function to calculate the distance between the two points and compare it with the configured value (which is passed to c3.generate method along with other config)

 Example

         point: {
             show: true,
             r: 5,
             sensitivity: 20
         },