Get rid of confusion between `index.js` and `wpcom+xhr.js`

- Since XHR is the default method for most users, the code to provide this default lives in `index.js`.
- The actual implementation of `WPCOM` is now in `wpcom.js`. This means you can do `require('wpcom/wpcom')(myRequestFunction)` to pass in your own implementation without any dependencies on XHR.

This change should not affect the public API or its behavior.