Slicer: flexible mosiacs of MRI images

Simple example:

```python
slc = Slicer("avg152T1_brain.nii.gz", "zstat.nii.gz", "mask.nii.gz")
slc.plot_activation(pos_cmap="OrRd_r", neg_cmap="PuBu")
```
![slicer](https://cloud.githubusercontent.com/assets/315810/3136650/f9a08c9e-e844-11e3-93a9-433349953a92.png)

More examples in this [IPython notebook](http://nbviewer.ipython.org/gist/mwaskom/890e45d29ebef1fcfe6b).