support proxy settings

Since you guys use superagent for your http requests, why not adding proxy support?

I have customers that allow outbound traffic only through their proxy. Which means, they cannot use Dropbox with my code at the moment.

Basically, just use superagent-proxy like this:

```js
var request = require('superagent');

require('superagent-proxy')(request);

downloadRequest = function (path, args, auth, host, accessToken, selectUser, proxy) {
. . .
apiRequest = request.post(getBaseURL(host) + path) . . .;

if (proxy) apiRequest.proxy(options.proxy);
. . .
```

The `proxy` parameter should be configured in the Dropbox constructor.

Thoughts?