Call method to get the access token should be configurable

I've created a melody for Instagram. The call for the access token should be POSTed (not GET)
It should be in the config (ex: app.yml)

```
// sfOAuth2::getAccessToken line 75 :
$params = $this->call($url, $this->getAccessParameters(), null, 'GET');  // Instagram needs POST call
```
