Put yt in its own file and add a quick-movie-maker tool

Like it says on the box.  

Example:

```
import pyregion
cube = SpectralCube.read('APEX_H2CO_303_202.fits')
scube = cube.subcube_from_ds9region(pyregion.parse('galactic\ncircle(0.66,-0.02,0.1)'))
ytcube = scube.to_yt()
ytcube.quick_render_movie('test_quickrender')
```

It works!  Doesn't look the best but that's OK.

Still needs docs!

cc @jzuhone @astrofrog @ChrisBeaumont