Added ability to create relationships

I love your tool, I find it very useful to prepare graphs for blog posts about neo4j, but one thing I was missing was the ability to create new relationships, so I decided to fork your repo and add this piece of functionality. This was my first time forking a repo, so I am not sure if I am doing everything right. Also I didn't have too much time to test it, but it seems to be working fine.

I added an ID field to the nodes, so when you double click on them you can see the node ID.

![screen shot 2014-01-09 at 14 19 02](https://f.cloud.github.com/assets/3496428/1877882/c915bca8-7930-11e3-8bb7-c3836f7d38e9.png)

Then, let's say you want to add a new relationship "R" between node 3 and node 5, all you have to do is click on the "+ relationship" button and enter 3 as the FROM node and 5 as the TO node. 

![screen shot 2014-01-09 at 14 19 24](https://f.cloud.github.com/assets/3496428/1877885/d0a5506e-7930-11e3-8350-b8d1cb4d4f0b.png)

I also added these fields in the edit relationship modal so you can change or move a relationship easily

![screen shot 2014-01-09 at 14 19 56](https://f.cloud.github.com/assets/3496428/1877886/d5282d46-7930-11e3-9264-ed38d3805b21.png)