Starting PromiseKit+StoreKit

Based on PromiseKit+UIKit (UIAlertView).
Fixed leak

Releasing the delegate on the productsRequest:didReceiveResponse:
leaves a delegate zombie, because requestDidFinish will be called
immediately.
Added rejecter on request:didFailWithError:
Add PromiseKit/StoreKit to the Podspec

Tag 0.9.7
Returning to main queue no longer necessary

Due to thenOn, and Zalgo avoidance, the queues we are on need to subsequently be on are handled in the `then`, all that is necessary is to ensure we do as much as possible in the background.
Handle text/* mimetypes

TODO extract encoding rather than assuming UTF8