Added compatibility with Brunch's command-line tests.

Although the `nodejs` branch provides standalone NodeJS test functionality, evidently all that's needed to get them working with Brunch (http://brunch.io) tests is to give canvasses a default style object. (And having `jsdom` and `canvas` installed, of course.)

Without this, trying to run Brunch tests involving Kinetic fail because Layer tries to set `this.canvas.getElement().style.position = 'absolute'` and evidently jsdom's implementation of canvas does not give it a default `style` object.

With this change, the style object is guaranteed to exist, and Brunch tests work.