Automatically add the provided client_id and client_secret

Automatically add the provided client_id and client_secret to the params when requesting a token.

Seems odd that if demanding client_id and client_secret in the constructor and providing get/set methods not to use that data within the lib itself. Rather than having to pass them manually like so:

```
$accessToken = $client->getToken(array(
    'client_id' => $client->getId(),
    'client_secret' => $client->getSecret(),
    'grant_type' => 'password',
    'username' => 'username',
    'password' => 'password'
));
```