add rdf mapping for blog posts


| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

this is a simple mapping for the blog post.

we could also change the template, but that would create a dependency on CreateBundle. this bundle probably should not provide a template anyways - the other bundles don't.

```
+{% createphp post as="rdf" %}
+
 <div class="row-fluid">
     <div class="span12">
-        <h1>{{ post.title }}</h1>
+        {{ rdf.title|raw }}
     </div>
 </div>
 <div class="row-fluid">
@@ -25,9 +28,11 @@
     </div>
 </div>
 <div class="row-fluid">
-    <div class="span12">
-        {{ post.body|nl2br|raw }}
+    <div class="span12" {{ createphp_attributes(rdf.body) }}>
+        {{ createphp_content(rdf.body)|nl2br|raw }}
     </div>
 </div>
+{% endcreatephp %}
```