Icon image loading related exports

This PR suggests making `ol.style.Icon#load` and `ol.style.Icon#getImageStyle` exportable. When using postcompose and the immediate API (`vectorContext`) the user is responsible for loading the icon image before calling `setImageStyle` and `drawPointGeometry` to actually draw the icon. By making these methods exportable we make this possible.

See the initial [request](https://groups.google.com/d/msg/ol3-dev/583OUseAdjg/iZk8On-xK7gJ) on the mailing list. And see this[jsfiddle](http://jsfiddle.net/B6Ld3/26/) for an actual use-case.