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.
Could you add a test for this code path? For that test, a SinonJS spy should be appropriate.