Added minimal support of the connectors, and prototyped class diagram related connectors.

There are some issues should be solved before further development:
1. The old version of umlsync was based on runtime attach of the diagram elements, but Marionette use buffering for this purpose.
One of the possible alternative to render and empty buffer and add child one by one, but it looks ugly.

2. It will be good to have some kind of status provide for the diagram loader: loading content -> content validation -> loading "class element" -> loading "interface element" ->  done
   Right now the list of requirements was required under initialize method of the diagram view