Add setStyle and getStyle to ol.Feature

As with vector layers and feature overlays, feature style can be set by calling
setStyle.  Calling getStyle returns what was passed to setStyle.  Internally, we call getStyleFunction.

examples/icon.js

@@ -18,19 +18,17 @@ var iconFeature = new ol.Feature({
   rainfall: 500
 });
 
-var styleArray = [new ol.style.Style({
+var iconStyle = new ol.style.Style({
   image: new ol.style.Icon(/** @type {olx.style.IconOptions} */ ({
     anchor: [0.5, 46],
     anchorXUnits: 'fraction',
     anchorYUnits: 'pixels',
     src: 'data/icon.png'
   }))
-})];
-
-iconFeature.setStyleFunction(function(resolution) {
-  return styleArray;
 });
 
+iconFeature.setStyle(iconStyle);
+
 var vectorSource = new ol.source.Vector({
   features: [iconFeature]
 });