Added line tracing support [clean]

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.

Note that I did not rebuild with grunt - I only edited the source file.

Secondly, this is a re-make of the original pull request #84, with a modified target branch.