Trans GUI

[not ready for review, replaces https://github.com/mne-tools/mne-python/pull/379]

Open issues:

1. [x] FIX:  For scaling, allow the bem file to have a different name from fsaverage 
2. [x] TESTs:  try to add TESTs for the GUI through traits
3. [ ] DOC:  Add documentation to the Python page
4. [ ] DOC:  Document traitsui backend issue and possibly handle with automatic default selection
5. [x] scale source spaces
6. [x] wait for and use https://github.com/mne-tools/mne-python/pull/739
7. [ ] add trans-fname parameter
8. [x] load existing trans file
9. [ ] automatically load trans when selecting raw and MRI?