Adds support for partner pre-population in authorize_url

Previously, unlike in other libraries, partners could not pre-populate details about a prospective merchant (e.g. email address, company name, type of business) using the `authorize_url` method - they simply had to append onto the generated URL. You can read more about the pre-population options [here](https://gocardless.com/docs/partner_guide#prepopulating-information).

The method now merges all the attributes passed into its `$options` parameter in the request, so it can support pre-population and any other options that we might see fit to add in future.

Now you can do something like this:

```php
<?php
// ...
$authorize_url_options = array(
  'redirect_uri' => 'http://localhost/~tim/gc/redirect.php',
  'merchant' => array(
    'name' => "Tim Rogers"
  )
);
print $gocardless_client->authorize_url($authorize_url_options);
?>
```

fixes #17