Add 'title' option to display a title on the left or right axes.

This is primarily designed for the graph2d line graph - I've not tested it against the timeline since I'm not currently using timelines (yet!).
Note that the example does show up bugs with dynamically changing options in the graph2d (as per #413). I've not looked at this as I know you're going to look into it, and I suspect that elements are not being removed, so there is probably a general fix required (ie re-render after option change which doesn't appear to be done unless orientation changes).
If this isn't implemented as you'd like, please let me know - I'd be happy to update as I'm quite keen to extend the graph2d.