Add exstra currency ids

Author: dpetrovics

src/clj/stripe/schema.cljc

@@ -43,9 +43,8 @@
      (s/either inner (Channel inner))))
 
 (def CurrencyID
-  "Currently only USD is supported, because we're located in the
-  US. Generalize if Stripe opens this up."
-  (s/eq "usd"))
+  "Right now just these two are supported."
+  (s/enum "usd" "cad"))
 
 (s/defschema Metadata
   "Metadata feature supported by the Stripe API. Keyword keys are converted to strings on the way over.