Added empty-room noise

Empty room noise and resulting covariance matrix. The empty-room noise was recorded on Dec. 6, 2002, while the sample dataset is from Dec. 3, 2002.  There is an artifact at about 115s, so I limited the length of the file to 110s (with the added benefit of having a smaller file).

The updated dataset is here:

ftp://surfer.nmr.mgh.harvard.edu/pub/data/MNE/MNE-sample-data-processed-0.7.tar.gz

Please have a look, I will move the file to the default location once this PR has been merged.