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 as a named optional parameter called `$page_handler`, 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:

```perl
# Old way (still works)
my $disputes = $bp->get_all($bp->get_disputes({
    limit           => 100,
    'created_at[>]' => '2014-05-01T00:00:00',
})->{disputes};
print 'number of disputes total: ' . @$disputes;

# New way, more flexible because it can display progress to the client and process records when they become available
$bp->get_disputes({ limit => 100, 'created_at[>]' => '2014-05-01T00:00:00' },
    page_handler => sub {
        my ($res) = @_;
        print 'number of disputes per-page: ' . @{$res->{disputes}};
    });
```