Trans GUI

[not ready for review, replaces https://github.com/mne-tools/mne-python/pull/379]

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:  Document traitsui backend issue and possibly handle with automatic default selection
- ENH?:  create source space before scaling