make buffer[index] work

This unfortunately took a lot of commits since `buffer[index]` only works with the underlying `SlowBuffer` and not with `Buffer`, so I had to merge all the method signatures from `Buffer` over top of the `SlowBuffer` implementations.

Despite the sheer volume of changes, all the existing tests pass plus some new ones. After I got all the tests to pass in node, I inlined all the comparisons with the built-in `Buffer` so that we can run the test suite in browsers.

I also included a "testling" field for the package.json so we can verify which browsers are currently working and which ones are failing.
Here's the status for my fork: https://ci.testling.com/substack/buffer-browserify

To add a hook for testling, add `http://git.testling.com` to the "WebHook URLs" section of [the server hooks page](https://github.com/toots/buffer-browserify/settings/hooks). Once the hook is working you can add a badge to the readme with:

``` markdown
[![browser compatibility](https://ci.testling.com/toots/buffer-browserify.png)](https://ci.testling.com/toots/buffer-browserify)
```