ENH: Plotting enhancements

This PR does three things:

1. Fixes error with label smoothing.

2. Adds option to make the STC overlay uniformly transparent by passing argument along to PySurfer.

3. Allows a user to create a label as `mne.label(vertices, hemi='lh')`. Presumably I'm not the only person who wants to be able to just plot a set of vertices on, say, fsaverage by doing:
```
label = mne.Label(vertices, hemi='lh')
label.smooth('fsaverage')
brain.add_label(label)
```
Currently, these additional lines must be used, since `mne.Label()` requires `pos` and `values` to be passed in:
```
val = np.ones(len(vertices))
pos = np.zeros((len(vertices), 3))
```
I think these make a sensible default so people can more easily plot labels that they create.