memoryConfig not optional for Storage.init as comment says.

``` /// Initialize storage with configuration options.
///
/// - Parameters:
/// - diskConfig: Configuration for disk storage
/// - memoryConfig: *Optional*. Pass config if you want memory cache
/// - Throws: Throw StorageError if any.
public convenience init(diskConfig: DiskConfig, memoryConfig: MemoryConfig, transformer: Transformer) throws {
let disk = try DiskStorage(config: diskConfig, transformer: transformer)
let memory = MemoryStorage(config: memoryConfig)
let hybridStorage = HybridStorage(memoryStorage: memory, diskStorage: disk)
self.init(hybridStorage: hybridStorage)
}
```

This says `memoryConfig` should be Optional, but it's not. Not sure which is the typo.