Allow default request options to be passed to DefaultTransporter

Problem: Certain (any?) portions of the library cannot be used easily from behind a proxy. While `googleapis.GoogleApis()` seems to accept a transporter parameter, parts of the library (`lib/auth/authclient` and `lib/requests` for example) use `DefaultTransporter` without allowing it to be overriden.

I suggest exposing `request.defaults()` function via `DefaultTransporter`, so that proxy can be configured for all subsequent Google API requests with a single call like this:

```js
googleapis.transporter.defaults( { proxy: 'http://my.proxy.ext/' } );
```

(This allows other default parameters to be passed to `request`, too, as per [request.defaults](https://github.com/mikeal/request#requestdefaultsoptions))