rename `add_merchant` to `promote_to_merchant`, leaving the old name as an alias. Accept either a merchant URI or a mapping object of merchant data.
include pending deprecation warning for the add_merchant method alias
fix test to reflect new language of the refund failure error coming back from the api
use the promote_to_merchant method where applicable in tests
Merge pull request #24 from jrus/master

allow either merchant URI or mapping of merchant data in "promote to merchant" method