ENH: Use multiple views simultaneously

This is not without bugs, but it should allow people to try using it to see what they think.

MNE folks (@agramfort, @dengemann), you can use my MNE branch that has some edits to play around with it:

https://github.com/Eric89GXL/mne-python/tree/pysurfer-fixes

The following code might get you working with it:
```
import mne
stc = mne.read_source_estimate(stc_fname)
brain = stc.plot('fsaverage', hemi='split', views=['lat', 'med'],
                 smoothing_steps=3, transparent=False,
                 time_label=None, fmin=-2, fmid=0, fmax=2,
                 colormap=mne.viz.mne_analyze_colormap([0, 1, 2]),
                 time_viewer=True)
```
For me, it produced this type of image:
![screenshot from 2013-07-23 14 12 48](https://f.cloud.github.com/assets/2365790/844951/bf945e52-f3dc-11e2-8033-fda008c91c83.png)