Design updates with error handling

lib/strike_me/api/account/balances.rb

@@ -21,6 +21,10 @@ module StrikeMe
         def to_a
           @items.map(&:attributes)
         end
+
+        def btc
+          @items.find { |item| item.currency == 'BTC' }
+        end
       end
     end
   end