FIX: Throw exception if >2GB

Addresses #923. I didn't add a test because I didn't want to write a 2GB file in the test suite.. a simple test is this:

    import mne
    from mne.datasets import sample

    data_path = sample.data_path()

    raw_fname = data_path + '/MEG/sample/sample_audvis_raw.fif'

    raws = [mne.fiff.Raw(raw_fname) for ii in range(20)]

    comb_raw = raws[0]
    comb_raw.append(raws[1:])

    tmp_dir = '/local_mount/space/megmix/2/users/mluessi/temp'
    comb_raw.save(tmp_dir + '/test_raw.fif', overwrite=True)

it would be great if someone else who is using a non-Linux platform (Mac, Win?) could also check if it throws an error  when the file size reaches 2GB.