2 new init methods LiquidPackage, Value and Variable

Signed-off-by: Miguel M. Almeida <miguelcma@gmail.com>
[Fix] return NSNull instead of nil (LQLiquidPackage)

A `[NSNull null]` was being returned instead of returning a `nil`.
This can cause a crash on the app. So, older versions of the SDK than this commit
should not receive a `null` from the server.

Signed-off-by: Miguel M. Almeida <miguelcma@gmail.com>