Balanced: Handle "pending" refunds

If the response from Balanced has a status of "pending", assume it
succeeded.

Closes #1221.
adding pag_seguro to README file
Immediately convert date fields to integers

While this was being done before validation previously, if an
unvalidated card was passed to a gateway it could end up having to deal
with Strings, which would entail type conversion sprinkled all over the
place.

This ensures that month, year, start_month, and start_year are all
converted when set, so that we can always assume those fields are either
an Integer or nil.

See #1214 for the motivation for this change.

Closes #1217.
Merge pull request #1222 from celsodantas/pag_seguro_to_readme

adding pag_seguro to README file