Add custom fields support

Add support for passing through custom fields to the checkout by passing a hash through to the checkouts partial

    render 'payola/transactions/checkout', sellable: sellable, custom_fields: {user_id: current_user.id}

The fields are signed with the payola secret key.

The hash can be retrieved using the Sale.custom_fields method

    config.charge_verifier = lambda do |sale|
        custom_fields = sale.custom_fields
    end