Fix the `proxy_authentication_handler` name

I'm sorry, but the docs say `authentication`, not `authentification`.[1] If you do what the docs say, you simply break your Couch: it starts responding with HTTP 500 to every request that is not authenticated by an earlier handler.

According to the Urban Dictionary, "authentification" is "authentication process which is not working".[1] You can't argue with the UD on that! :)

Some people using this handler might actually use the wrong spelling in their configs, so I left the original name as an alias.

[1] http://docs.couchdb.org/en/latest/api/server/authn.html?highlight=authentication#proxy-authentication
[2] http://www.urbandictionary.com/define.php?term=authentification