Expand Label to surface using patch info

Following up on issue https://github.com/mne-tools/mne-python/issues/1180, adds the following modifications:

 - in the `Label` constructor, add the option to expand a label from a source space to the entire surface (using patch info)
 - add a `Label.fill()` method to expand a label defined in source space to the surface
 - adds to `stc_to_label` the option to expand the label using patch info
 - adds to `setup_source_space()` the `add_dist` option to call `add_source_space_distances` on the source space before returning it, with a warning that the default is going to change to True in 0.9 
actually now that you pointed it out maybe I should also rename `src` here to `fwd`