FIX: Refactor cov, projs, evoked, etc.

This PR does a few things:

1. Moves `Evoked` / `read_evokeds` / `write_evokeds` into `mne` namespace.

2. Merges `mne/io/cov.py` into `mne/cov.py`.

3. Updates the doc to be clearer about functionality split between raw and non-raw IO.

Note that I tried to merge `mne/io/proj.py` into `mne/proj.py`, but it was really difficult because things within `io` and outside `io` both critically depend on proj functionality. If we want to merge them, we'll need to create some other place to put all the `fiff` routines, and do a bunch of refactoring (not fun).

Tests pass over here, so should be ready for review.