Hide .appveyor.yml with dotfiles, update links

[hugovk]

Rename appveyor.yml to .appveyor.yml to hide it with the other dotfiles, and update some links.

(Same as #226 but without dropping Python 3.4.)

[blueyed]

codecov.yml → .codecov.yml

Codecov's docs say that they only support codecov.yml officially, so I'd leave that as-is.

[hugovk]

Good spot!

They used to support both codecov.yml and .codecov.yml:

• https://web.archive.org/web/20190430062713/https://docs.codecov.io/docs/codecov-yaml#section-can-i-name-the-file-codecov-yml-

But now only codecov.yml:

• https://docs.codecov.io/docs/codecov-yaml#section-can-i-name-the-file-codecov-yml

Reverted.

[blueyed]

Has conflicts now, please rebase.

[hugovk]

Rebased!

[blueyed]

Thanks! :)