Support for Atlas as a datasource

This is still a rough cut but at least the default dashboard loads. Time shifting and multiple lines seem to work as well.

Config spec looks like this:

```js
atlas: {
  type: 'atlas',
  url: "http://localhost:7101"
}
```

Default targets definition looks like this:
```json
"targets": [
  {
    "query": "(,0h,1w,),(,name,sps,:eq,nf.cluster,nccp-silverlight,:eq,:and,:sum,:swap,:offset,),:each"
  }
],
```

See https://github.com/Netflix/atlas for info on Atlas.

Some things to sort out:

- [ ] Data source dropdown goes blank
- [ ] Metrics editor goes blank
- [ ] Test WoW legend support
- [ ] 5m window defaults to 0.1s step size which Atlas doesn't support
- [ ] PNG display