ENH: use straight subprocess.call instead of 'handling' stout/stderr

pros:
- no buffering on output
- probably would make things easier to debug since no stdout/stderr redirection

cons:
- no 'beautification' of stdout to replace mne_ with mne and stripping .py suffix

Also exits with correct return code returned by the call (could have been done
in original version too)
DOC: adjusting usage examples to use 'mne command' instead of 'mne_command.py'
ENH: centralize definition of OptionParser to deduce command name and set version, etc fields

With changes to mne_surf2bem as a demo "case"
finish the cleanup from @yarikoptic in mne_* commands