Difficulty selectbox in Studio (based on new XBlockAside functionality). Include:

- adaptation asides to be imported from the XML
- updating SplitMongo to handle XBlockAsides (CRUD operations)
- updating Studio to handle XBlockAsides handler calls
- updating xblock/core.js to properly init XBlockAsides JavaScript

cms/templates/structured_tags_block.html

@@ -1,7 +1,8 @@
 <div class="xblock-render">
     % for tag in tags:
     <label for="problem_tags_${tag['key']}">${tag['title']}</label>:
-    <select id="problem_tags_${tag['key']}" name="problem_tags_${tag['key']}">
+    <select id="problem_tags_${tag['key']}" name="${tag['key']}">
+        <option value="" ${'' if tag['current_value'] else 'selected=""'}>Not selected</option>
         % for k,v in tag['values'].iteritems():
             <%
                 selected = ''