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)
```

Left to do based on @mwaskom and @agramfort comments:

- ~~Figuring out how to do some things with it was a process of trial and error that called on a lot of existing knowledge about how things work behind the scenes.~~ *This will have to be more specific...*
- ~~The examples showing "split" doesn't work~~
- ~~bug: brain.save_image('dSPM_map.png') only one image is saved.~~
- ~~bug: brain.screenshot() only one image is loaded.~~
- ~~In cases like loading an annotation or morphometry when viewing "both" hemis, it might be nice to try and load both files, and then excepting if that fails (saying the hemisphere needs to be specified).~~
- ~~bug: brain.show_view('lateral') all brains with rotate to lateral view~~ *@agramfort I couldn't replicate this, you should have to specify the row, col indices (default -1, -1).*
- ~~None of the examples document hemi="both"~~
- ~~The narrative docs on the config file and visualization customization need to be updated.~~
- ~~Using "both" or "split" as a hemi from the command line interface doesn't work~~
- ~~The hemi="both" option doesn't appear in the Brain docstring~~
- ~~Since "split" and "both" aren't constructs anyone from the Freesurfer world are going to know about, some more explanation about what's going on there in the Brain docstring itself would be good.~~
- ~~In quite a few of the examples, the brains are in the wrong place in the image~~
- ~~We'll definitely want an extensive "what's new" file.~~
- ~~For some reason the pysurfer windows weren't closing when I was buiding the docs. They did at the very end, but it was a little annoying as that was happening. If this reflects a deeper problem with closing figures when using a script, it definitely needs to be fixed.~~
- ~~Someone needs to add [me] to the authors list on the homepage~~