add buffer package for use in browser

[rvagg]

Ref #50

This is only used in the browser, Node.js will default to core buffer, so the main cost of this is the dependency weight, but it adds nice future proofing for the browser and Webpack.

[calvinmetcalf]

I'm leaning more towards #53 since we have other cyrpto-browserify prackages that this one depends on that use safe-buffer so using it would add anything to the dependencies and until we can switch all of the crypto-browserify stuff to use the correct thing