ENH: Add interpolation code and example

cc / FYI @agramfort @wmvanvliet @MadsJensen @t3on @christianbrodbeck 
If you're getting rid of the "head"-ness, might as well call this `origin`. Also, it should probably be in some other location. Even `mne.channels` somewhere is a good place for it, and then you can import it in this file and avoid one of your nested imports in `mne.channels.interpolation`.