Replace `distance_matrix` with `cdist`

Here's a start:

```
DiPy's bundles_distances_mam(): 1.04474496841 sec.
Python loop + avg_mam_distance_dipy: 1.36809921265 sec.
Python loop + NumPy + Scipy: 6.4115049839 sec.
Python loop + NumPy: 13.3757660389 sec.
Python loop + NumPy (faster implementation): 10.7866270542 sec.
```