divide by 2 memory usage of read_raw_brainvision

problem noticed when playing with:

ftp://ftp.fcdonders.nl/pub/fieldtrip/tutorial/SubjectEEG.zip

see http://fieldtrip.fcdonders.nl/tutorial/continuous

My script:

```
import mne

raw = mne.io.read_raw_brainvision('subj2.vhdr', preload=True, eog='64')

events = mne.find_events(raw)

mne.viz.plot_events(events, raw.info['sfreq'])

events = mne.merge_events(events, [111, 121, 131, 141], 1000)  # animals
events = mne.merge_events(events, [151, 161, 171, 181], 2000)  # tools

event_id = dict(animals=1000, tools=2000)

reject = dict(eeg=200e-6, eog=200e-6)

epochs = mne.Epochs(raw, events, event_id, tmin=-0.2, tmax=.8, reject=reject)

evoked = epochs.average()

evoked.plot()
```

cc @t3on @christianbrodbeck 

@dengemann @wmvanvliet  dataset  to demo montage stuff?
yeah for branching below. If len(self._events) then I'll use next ch_idx to put stim.