Don't include the Buffer polyfill in dist/ files.

In a browser, there are multiple implementations of nodejs' Buffer. If we let browserify includes its own, we won't play nicely with the others (it uses `instanceof` and doesn't accept other implementations).
With this patch, we don't include any implementation but we let the environment provides (or not) the Buffer.

This pull request includes #140 (fixing #139 too) and the fix I prepared. I got no response from @humphd so I assume he's ok with it :)