Continue work on various currency support

Setup a hook_requirements() check to see if enabled currencies in
Commerce are supported by BitPay.
If no setting is provided, use the Commerce default currency. Don't
give users the option of defining what currency to tell BitPay through
config. That should come from elsewhere.
Found commerce.currency.inc already implemented most currencies and
their decimals, so removed them from this module hook_currency_info()
and just did Bitcoin instead. Replaced the list of supported BitPay
currencies.