Fixed server check, callbacks, and moved config params to a setup call in stead of a header file

I have been using MKStoreKit for several years. Thank you for producing it! I use it with both server side receipt validations and the free-for-app-reviewers feature. This pull request is to incorporate mostly minor changes to get those items to work for me. I encode the receipt with Base64 encoding as described in Apple's docs for server-side validation, pass along a Boolean based on the YES/NO response, and fixed a few callback signatures.

One slightly more ambitious update was to move the app-specific config parameters into a setup call in stead of having them in a header file, making this kind of sharing a bit easier as no code changes from project to project.

Thanks again for a great addition to the iOS development experience!