Added line tracing support

Line tracing support has been added.  The majority of the changes take place in the plotHover function, and revolve around a secondary tooltip calculation should the cursor not be on a data point.  I've added a new options item: an object called 'lines' that has members 'track' (bool, default false) which determines if the lines between points are tracked, and 'threshold' (number, default 0.05) which determines at what distance from the line the algorithm determines the cursor is hovering over it.

I am not familiar with grunt.js, and I am at the end of my work day so I will not be building the production .js file.  I was hoping you could do such for me if you agree to merge this pull request.