Seriously broken payment redirect changes

Add some code to do iframe on the payment redirect, copied from
uc_bitpay.
Add a settings form option to choose between redirection to hosted
payment or to an iframe page.
Possibly fix the rounding bug reported at
https://drupal.org/node/2091673#comment-7900475.
Use the currency code from the order for the BitPay query instead of
the default one from the site.
Attempt to save the remote status returned from the invoice creation as
part of the transaction initialization.