ENH: codespell support and related fixes

One can now do `make codespell` from the mne-python root directory to run codespell. I did, and these are the related changes. To install codespell, I did:

```Bash
$ git clone git://github.com/lucasdemarchi/codespell.git
$ cd codespell
$ make install prefix=~/.local
```

Note that this includes a slightly customized dictionary, since we want options for things like using `didn\'t` instead of `didn't` to replace `didnt`. We also had some `deviato`'s in there, so I added an entry for `deviation`.