Added shipping cost estimate method, updated `requirements.txt` to fix Crypto import

[RagingRoosevelt]

Addresses #15 and #16

1. Adds Shopgoodwill method get_item_shipping_estimate that calls the /itemDetail/CalculateShipping endpoint with an item_id and zip code to estimate shipping cost. This is a little dirty since that endpoint returns partial HTML from which the price has to be extracted. No clean JSON return for this. Keeping in theme with @scottmconway's "what they hell were they thinking" observations.

2. Updates the requirements.txt entry for Crypto with pycryptodome to address some import problems that come up with pycrypto or other choices for package fulfilling the Crypto need.

[scottmconway]

LGTM. Thanks for your contributions!

[RagingRoosevelt]

Thanks for the really well put together module!

I'd been trying to write something similar and had gotten stuck on the login stuff when I found your work. Always fun to learn some new stuff along the way, too!