add missing hashes, clean up code

this pull does 2 things and can be broken up into 2 pulls if needed

1. adds sha224 and ssh384
2. cleans up the whole buffer factory setup and all of the hashes export themselves instead of a factory
They are in two commits so I can pull them apart

In index.js:

> @@ -1,12 +1,12 @@
>  var exports = module.exports = function (alg) {
> -  var Alg = exports[alg]
> +  var Alg = exports[alg.toLowerCase()]

LGTM

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