Add ability to pass sort and filter parameters to the 'all' method

A possible solution for issue #11.

Allow a hash of options to be passed to the `all` method. This means we can use [Paymill's listviews sort and filter options](https://www.paymill.com/de-de/dokumentation/referenz/api-referenz/index.html#listviews) when returning lists of objects.

For example to find the most recent transactions belonging to a client we can do:
    
    Paymill::Transaction.all({client: "client_id", order: "created_at_desc"})