Lazily create ol.defaultFeatureStyleFunction

Besides the slight performance benefit of not prerendering the default
circle style until we need to, this change also allows loading OL3
in browsers that don't support the canvas API (IE 7-8). If the circle
style is rendered on load, the lack of HTMLCanvasElement#getContext
causes IE 7-8 to bomb, regardless of if vector styles are used at all.
Merge pull request #1815 from Intermedix/lazy-style-function

Lazily create styles returned by ol.defaultFeatureStyleFunction.