Basic support for text symbolizers

This adds basic label rendering for points and polygons to the
canvas renderer, and a text symbolizer to the style package.

examples/vector-layer.js

@@ -1,6 +1,8 @@
+goog.require('ol.Expression');
 goog.require('ol.Map');
 goog.require('ol.RendererHint');
 goog.require('ol.View2D');
+goog.require('ol.filter.Filter');
 goog.require('ol.layer.TileLayer');
 goog.require('ol.layer.Vector');
 goog.require('ol.parser.GeoJSON');
@@ -10,6 +12,7 @@ goog.require('ol.source.Vector');
 goog.require('ol.style.Polygon');
 goog.require('ol.style.Rule');
 goog.require('ol.style.Style');
+goog.require('ol.style.Text');
 
 
 var raster = new ol.layer.TileLayer({
@@ -27,6 +30,19 @@ var vector = new ol.layer.Vector({
           strokeColor: '#bada55'
         })
       ]
+    }),
+    new ol.style.Rule({
+      filter: new ol.filter.Filter(function() {
+        return map.getView().getResolution() < 5000;
+      }),
+      symbolizers: [
+        new ol.style.Text({
+          color: '#bada55',
+          name: new ol.Expression('name'),
+          fontFamily: 'Calibri,sans-serif',
+          fontSize: 12
+        })
+      ]
     })
   ]}),
   transformFeatureInfo: function(features) {