Allow to provide parameters for PullRequests list

According to the [API documentation](https://developer.github.com/v3/pulls/#list-pull-requests) of the v3 API a sort can be added and some other parameters not yet supported by this library. I have changed the "all" function to allow parameters to be given.
add `$defaults` array containing basic values from constructor and merge it

```php
<?php

$parameters = array_merge(array(
    'page' => 1,
    'per_page' => 30
), $params);
```