Use signature instead of certificate when testing paypal as recommended
by them.
Remove calls to depricated methods in paypal test.
Don't include the depricated field "Name" with the billing address.

The name on the CC is included elsewhere as "FirstName" and "LastName".
Including both was causing some issues was causing some strange
behaviour.
Revert "Don't include the depricated field "Name" with the billing address."

This reverts commit a4170d97ed85e474b44cf87b45f1f8eda15a09c8.
Use FirstName and LastName instead of Name in payflow.
Directly map first_name and last_name instead of combining them into one 'name' in payflow_link
Merge pull request #599 from Shopify/paypal-double-name-fix

Paypal deprecated name field fix.