Parse XModuleDescriptors on import using from_xml

Also:
Render all XModuleDescriptors in the cms the same way
Default them to editing raw xml, if there is no specific module for them

common/lib/xmodule/js/module/raw.coffee

@@ -0,0 +1,9 @@
+class @Raw
+    constructor: (@id) ->
+        @edit_box = $("##{@id} .edit-box")
+        @preview = $("##{@id} .preview")
+        @edit_box.on('input', =>
+            @preview.empty().text(@edit_box.val())
+        )
+
+    save: -> @edit_box.val()