ENH: keep old ref_channels names

I erroneously renamed the ref channels according to a simple and
logical scheme that effectively blocks using adding the weight tables
using mne_creacte_comp_data and mne_add_to_meas_info.
Keeping the old name makes it less painfull to add the weights
ENH: drop low level reading
COSMITS
FIX: restore previous naming
Merge pull request #470 from dengemann/bti_fix_read

ENH: drop os low level file handlers

Just a maintenance PR, good to go.