Fix to filter empty room before calculating cov.
Simpler way to construct filename.
Include option to filter empty room files.
Minor filename fix and cosmits.
Merge pull request #38 from pravsripad/empty_room_filt

Fix to filter empty room before calculating cov. The application of filter is optional. Empty room files are saved as '-empty.fif' and covariance files as '-cov.fif'. Empty room cov matrices are saved as '...,empty-cov.fif'.