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.html

@@ -37,7 +37,7 @@
 
         <div class="span4">
           <h4 id="title">Vector layer example</h4>
-          <p id="shortdesc">Example of a countries vector layer with country information on hover.</p>
+          <p id="shortdesc">Example of a countries vector layer with country information on hover and country labels at higher zoom levels.</p>
           <div id="docs">
             <p>See the <a href="vector-layer.js" target="_blank">vector-layer.js source</a> to see how this is done.</p>
           </div>