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
the problem with deprecating 'emprical' now is that 'keep_sample_mean' is only allowed with 'empirical' right now. you will have a failing test against C if you don't use empirical. Maybe we can decide about this after merge.