Composer for graph targets

This adds a simple JSON editor for the target definitions of a graph. Notable features:

* previews graph whenever a change is made
* however, it only updates the preview if...
  * the JSON data passes jsonlint
  * Graphite returns a 200 success to our HEAD request for the new url
* disables the submit button when either of the aforementioned failure cases are active
* bound to `alt+cmd+enter` or `alt+ctrl+enter` combo keys (UI button forthcoming)

![screen shot 2013-06-14 at 1 21 21 pm](https://f.cloud.github.com/assets/494338/655588/02f108f6-d517-11e2-892c-f0b7bdcb28a3.png)