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
Why even allow a callable? I think 99.9+% of users will never use this. If you need it for your own testing, it's fine to leave the capability in there, but I'd leave it as an undocumented feature. And from the docstring I don't know what `None` does...?