added support for batch creation of gift cards. need to write tests

I haven't written the tests yet because I can't get the tests to run and would need some direction. Right now, this code is living in `vendor/spree_gift_card` in my spree app. When I run the rspec tests from the app it doesn't get these tests and if I try and run the tests in `vendor/spree_gift_card` it doesn't work because it isn't a running environment. Any advice?

Also, this is my first attempt at sharing my updates into an existing rails application so if you notice any newbie issues with style or code please let me know.