Added missing update functionality

[Sega-Zero]

As I mentioned in #103, there is a proper way to update values in keychain - SecItemUpdate function.

This pull request adds a new method updateInSecureStore. Instead of deleting and adding after, Locksmith.updateData now uses the new updateInSecureStore method. Delete&Add is error prone, IMHO, I've seen the cases, when deletion returned errSecSuccess, but the value still persisted in keychain. Which is why I, usually, set the value to empty and then delete it.

I didn't included the tests from #103, but if you would like to merge it too, the User class should be changed to something like this in order to pass the test:

public func store() -> Bool {
        do {
            try self.createInSecureStore()
        } catch LocksmithError.Duplicate {
            guard let _ = try? self.updateInSecureStore() else { return false }
            print("succesfully updated a value")
        } catch let error {
            print("Couldn't store: \(error)")
            return false
        }
        return true
    }

Not sure if catching .Duplicate is a good way, maybe one should implement find method to ensure no value exists before trying to call createInSecureStore