Allow RNDecryptor to accept custom settings

Addresses the issue in #49: it's not possible to use custom RNEncryptorSettings that vary within an app, because RNDecryptor always uses kRNCryptorAES256Settings.

I've added two new class methods that take a settings argument, and used a property to hold the settings until they're needed. This matches the public API of RNEncryptor, and is similar to the API that RNOpenSSLDecryptor exposes. It should be backwards compatible and transparent to users of the library who don't want to vary their settings.