ENH:  Option to fix psychtoolbox/stimtracker events

Option to fix events from a vmrk file from a recording where triggers were
sent by psychtoolbox and recorded through the stim tracker (KIT setup)
STY
OPT:  make KIT and brainvision readers to share read_elp

(move read_elp to mne.coreg)
TEST:  read_elp
FIX:  initial file offset when reading from brainvision eeg