IPython paths

Fetches expected IPython paths for you, both system installed and user installed
kernels.

Only works on IPython 3.x. For 4.x, you'll need jupyter-paths.

For anything earlier, send a PR.

$3

``javascript
> require('.').kernelDirsListing()
[ '/usr/share/jupyter/kernels',
'/usr/local/share/jupyter/kernels',
'/Users/rgbkrk/.ipython/kernels' ]
`

$3

`javascript
> require('.').kernelDirs().then((dirs) => console.log(dirs))
Promise { }
> [ '/usr/local/share/jupyter/kernels' ]
``