WIP: MRI/Head Coregistration / Scaling FsAverage

Following up on https://github.com/mne-tools/mne-python/issues/369

Open Issues:
- FIX
 - For scaling, allow the bem file to have a different name from fsaverage 
- TESTs
 - try to add TESTs for the GUI through traits
- DOC
 - integrate the documentation page
- ENH
 - set the qt backend as default for traitsui
- ENH (possibly for the future)
 - create source space before scaling