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(
    'grant_type' => 'password',
    'username' => 'me@willpillar.com',
    'password' => 'minor101.'
));''