Make the default security stronger

In SSKeychain's doc it says for the AccessibilityType: `If the value is NULL (the default), the Keychain default will be used.`
But I couldn't find in the apple docs what the default is and since this is a wrapper to make it easy for the user/developer I strongly think we should make it as save as possible and Apple recommends as well to always set `kSecAttrAccessibleWhenUnlocked` if possible.