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.
You'll be hard-pressed to access the default internal `remove` method from the tests. That's okay, though, because (for the default implementation) what you really want to do is ensure that the element is removed from the document (not that any particular JavaScript function was invoked)