Add a new field edit box with add

## Story

* https://www.pivotaltracker.com/story/show/75383400

## Notes

* Used a label that says "Edit this" because I thought a "+" would be too ambiguous (as it's used elsewhere)

## Screenshot 

![image](https://cloud.githubusercontent.com/assets/2136286/4239190/e9696210-39de-11e4-80b7-be951c5bba36.png)