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
Yeah, it's just weird to have `auto` not include all the available methods. Adding it to `auto` seems like a simple solution. However if you do decide it's not worth having it, then mentioning in the docstring (maybe in Notes) why one method or some methods are excluded from "auto" would also work.