Support curveTo

[leethomason]

A simple, initial implementation of curveTo(). A couple areas of improvement are in the notes; I'm happy to tune the function, but wanted to get unblocked from the API not being present at all.

[jonathanrpace]

Great! Don't suppose you fancy tackling cubicCurveTo while you're there? :)

One suggestion - is it worth passing the 'numDivisions' as an optional parameter, with a default value of 8? That way if people need to tweak it, there's no need to open up the class.

[robsilv]

Thanks Lee, your contributions are obviously very welcome! :D

[leethomason]

@jonathanrpace, Good reminder on the cubics. Beginning of the week I'll add the cubics, and also add a proper error metric for the quadratics so it can compute a reasonable number of "numDivisions" for you. That will be much better than the current hardcoded value, and if we stiff need tuning that can be added later.

@robsilv You are welcome - glad to see this getting momentum!