Keychain accessibility

[tinus-github]

Thanks for your work on this library and the associated app!

As far as I could see, you do not specify a value for the kSecAttrAccessible attribute, which specifies the policy for the keychain items used to store the passwords. Therefore it isn't really clear what happens. The result is probably kSecAttrAccessibleWhenUnlocked, which means the values are accessible when the device is unlocked and are backed up when using an iTunes encrypted backup, which is the behavior I want. But I would like to be sure.

Unfortunately I don't have spare iOS devices to test.

Could you please add an explicit value for this attribute so it is clear what happens and so it doesn't change if Apple decides to change the default?