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
no, not really. it has to be ordered (by conditions) and we need equal samples from each category. As mentioned earlier this will be an advanced example and will be moved to a dedicated section.