Add variable dot colors

I added two delegate methods to the `JBChartDotsViewDelegate` which call two corresponding delegates in the public `JBLineChartViewDelegate` to allow each dot on the graph to be a different color.  As is the convention for other color methods, one method is for choosing the color when not selected and the other method is for choosing the color when the line is selected.  If one or both of these delegate methods are not implemented, the old behavior is used.  That is, dots have the same color as the line.

Variable dot colors is useful for quickly showing another piece of information.  In my case, I'm showing orange if the day's goal was not met and green if the day's goal was met:
![img_3824](https://cloud.githubusercontent.com/assets/1313618/2684122/a13c712e-c1b3-11e3-8bb9-a02c58d87826.PNG)