dask.imperative requires toolz, but it's not listed as one of the setuptools "extras"

```
$ pip install dask[imperative]
Requirement already satisfied (use --upgrade to upgrade): dask[imperative] in ./env/lib/python3.4/site-packages
  dask 0.8.0 does not provide the extra 'imperative'
```

But base `dask` doesn't require `toolz`, and `dask.imperative` imports `toolz` on line 9.

Could you add a setuptools "extra" for "imperative"?
