Mixed source space

Code to generate source spaces with cortical surfaces and sub-cortical volumes.
Thinking about the API... I think I'd prefer the following over this function (which requires a source space to originally exist):
```
surf_src = setup_source_space(...)
vol_src = setup_subcortical_source_spaces(...) [MOST OF THIS FUNCTION]
src = surf_src + vol_src
```
Because it shows people how easily source spaces can be combined (because they're just lists). That way if someone wants a volume source space for the cortical surfaces or even no cortical surfaces (yikes!), they can do it. @agramfort WDYT?