Improve documentation and tests

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# Changes to be committed:
#	modified:   Changes
#	modified:   MANIFEST
#	modified:   README.md
#	modified:   cpanfile
#	modified:   lib/SilverGoldBull/API.pm
#	modified:   lib/SilverGoldBull/API/Response.pm
#	new file:   t/01-get_currency_list.t
#	new file:   t/02-get_payment_method_list.t
#	new file:   t/03-get_shipping_method_list.t
#	new file:   t/04-get_product_list.t
#	new file:   t/05-get_product.t
#	new file:   t/06-get_order_list.t
#	new file:   t/07-get_order.t
#	new file:   t/08-billing_address.t
#	new file:   t/09-shipping_address.t
#	new file:   t/10-item.t
#	new file:   t/11-create_order.t
#	new file:   t/12-quote.t
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