Fixed compatibility with phonegap 2.7.0

- Changed interface 'Phonegap.exec' to 'cordova.exec'
- Use paymentWithProduct instead of paymentWithProductIndentifier
because It was deprecated in iOS 5.0
- Changed interface of requestProductData parameters
- Add prefix in NSLog
Fixed a readme file
Merge pull request #1134 from davidshimjs/master

InAppPurchaseManager compatibility with Phonegap 2.7.0