Related vote in tree

Backend for giving details about the related vote in tree:

Example call from angular: /proposals/20/related_vote_in_tree.json
Example response from rails for an existing vote:
![existing_vote](https://f.cloud.github.com/assets/80631/505320/b7fff0f8-bd1c-11e2-87ee-539f89a5b2af.png)

Example response from rails when no existing vote: {} ( empty hash )