Adds upstream dependencies option to visualizer

Uses new api call /api/inverse_dep_graph/<task_id>, which returns
the same data types as  /api/dep_graph, except the "deps" field
is inverted.

E.g. If A requires B, the data returned is
```
{
  "B()": {
     deps: ["A()"]
  }
}
```

The UI changes are kinda tacked onto the existing dependency visualization, not really my strong suit :P

### Task Hierarchy:
![screen shot 2014-03-04 at 1 53 10 pm](https://f.cloud.github.com/assets/410962/2324888/620bfb6c-a3ce-11e3-90aa-ed6fbcb33e11.png)

### Current visualization of C()

![screen shot 2014-03-04 at 1 53 15 pm](https://f.cloud.github.com/assets/410962/2324934/bfbaa81c-a3ce-11e3-90a6-1bf7dd9cfab7.png)

### Upstream visualization

![screen shot 2014-03-04 at 1 53 21 pm](https://f.cloud.github.com/assets/410962/2324939/c7038b3e-a3ce-11e3-9617-dfc3c6424600.png)