Assert that an added layer has .insert function

[garrensmith]

Hi,

First off, I really like d3.chart. Great work and nice idea.
As a new user of this library I made the mistake of not adding an insert function to a layer. It took me a while to debug the problem.

I think it would be a good idea to add an d3cAssert in Layer.prototype.draw to make sure that this layer has an insert function.

If you are happy with this idea, I can send a PR.