Initial atlas datasource

Code copied from https://github.com/grafana/grafana/pull/1265

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: 'AtlasDatasource',
  url: "http://localhost:7101"
}
[...snip...]
plugins: {
  dependencies: ['grafana-plugins/datasources/atlas'],
  // Optional, used to run specs from grafana's test harness
  // requires https://github.com/grafana/grafana/pull/1272
  specs: ['grafana-plugins/specs/atlasDatasource-specs']
}
```

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:

- [x] Data source dropdown goes blank
- [x] Test WoW legend support
- [x] 5m window defaults to 0.1s step size which Atlas doesn't support
- [x] Metrics editor goes blank