podspec with optional components as subspecs

ToDo: Add OpenSSL static library to AppReceiptVerificator subspec. See:
http://stackoverflow.com/questions/19481125/add-static-library-to-podspe
c
Wrap NSCAssert with NS_BLOCK_ASSERTION to prevent unused variable warning
Missing source files
Merge pull request #23 from xgil78/AutoRenewableSubscription-tests

RMAppReceipt test
Bug fix: wrong object on product request notifications

Plus whole lotta of unit test assertions so this never happens again.
Improve performance by caching keychain transactions
Include OpenSSL in podspec
Single quotes
Include OpenSSL license
Test isActiveAutoRenewableSubscriptionForDate NSAssert
Log keychain errors instead of using NSCAssert
Log JSON errors instead of using NSAssert
Return an empty dictionary if JSON parsing fails
Increase version to 0.4.1
Bug fix: transactions cache was always nil until persisting the first transaction
Unit tests now fail if cache fails
Update README.md
RMStoreErrorCodeUnableToCompleteVerification. Fixes #31.
Use RMStoreErrorDomain