Add instructions for how to load datasets, objects in command line

Here's the stdout stream for running KCenters from the command line, using this PR

```
$ msmb KCenters --n_clusters 100 --inp atom_pairs --transformed kcenters --out model.pkl
KCenters(metric='euclidean', n_clusters=100, random_state=None)
*********
*RESULTS*
*********

KCenters clustering
--------------------
n_clusters : 100
metric     : euclidean

Inertia       : 7689.59786338
Mean distance : 0.0768967476013
Max  distance : 0.111259981904

--------------------------------------------------------------------------------
Saving transformed dataset to 'kcenters/'
To load this dataset interactive inside an IPython
shell or notebook, run

  $ ipython
  >>> from mixtape.dataset import dataset
  >>> ds = dataset('kcenters/')

Saving "model.pkl"... (<class 'mixtape.cluster.kcenters.KCenters'>)
To load this KCenters object interactively inside an IPython
shell or notebook, run:

  $ ipython
  >>> from mixtape.utils import load
  >>> model = load('model.pkl')
```