Adds remove method to layer (#63).

First-pass implementation. Seems to work (I removed the `.remove` call in the bar chart example, and the bars were removed), but obviously I need to add some tests.

Not super familiar with Sinon. I tried to spy on the `layer`'s remove method, but I couldn't get it working.
Is there a more elegant way to get this? `this.layer.selectAll('g')` will return a length-1 array even in after the elements have been removed, since it's the selection length rather than the length of the actual DOM elements.