Added autopagination helper with callback support

This standardizes the `get_all` method to return a HashRef as well (instead of an ArrayRef) to standardize the method interfaces, and adds support for automatic pagination of records.  By passing in a callback subroutine, the method modifier is able to automatically fetch all of the records and process them as they become available, and reduce the need for chaining `get_all` manually to the `get_disputes` method.