MRG: add option for automated regularization to mne.cov.compute_covariance

Todo:

- [x] show basic sensor examples / update existing examples
- [x] consolidate API including default values
- [x] deal with projs
Perhaps we change the default in a couple releases to `'auto'` or whichever method you found to be the best. We could immediately deprecate `method='empirical'`, and only have to wait one release cycle. WDYT?