Fancier brain plots using ipyvolume

Let's add `ipyvolume` to the requirements. We can then add a `plot_volume` function to Brain objects and Nifti objects. For example:

```
import ipyvolume as ipv
gray = se.load('gray')
ipv.quickvolshow(gray.get_data(), lighting=True)
```

Other ideas:
- It looks like, by default the volume rendering is flipped 90 degrees...we'll need to figure out how to rotate the camera to correct for that
- we could use this volume rendering to make animations. i think they'd loop pretty neat