Change log:
1. Added `get`, `add`, `remove`, `set` methods to the `Client` class
2. Created concrete selector classes (`UserListSelector` and `UserListClientSelector`)
3. Create Soap instant objects in Client class instead. As a result, `Operation` and selector class does not need a client instance before method `build` is called.
Breaking changes:
1. The `Selector` class has become internal. Concrete classes `UserListSelector` and `UserListClientSelector` should be used instead.
2. Selector classes constructors do not require a client object
3. Client object will build the selector internally. There's no need to call the `build` method on selector objects.