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
It's weird `ledoit_wolf` is not part of `auto`, I would assume `auto` would have all methods. I assume it's because it's not very good, in which case why have `ledoit_wolf` at all?