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

Todo:

- [x] show basic sensor examples / update existing examples
- [ ] consolidate API including default values
- [x] deal with projs
Hmm, I assume there is going to be more documentation on this somewhere? Currently I would not know how to specify these parameters, even if I wanted just a simple diagonal_fixed method. Or is it assumed that normal user should just use "auto"?