Prepend TableDirector mutable functions with @discardableResult

[Wisors]

Swift 3 version produces warnings about unused return values. For example, you need to change next function

    open func append(section: TableSection) -> Self {

        _ = append(sections: [section])
        return self
    }

to

 @discardableResult  open func append(section: TableSection) -> Self {

        _ = append(sections: [section])
        return self
    }

[maxsokolov]

Fixed.