Create addLabel method for Node class

[stevenmaguire]

Occasionally you need to add a single Label to a Node and it is clumsy to do so by first creating a new array.

I am proposing a new method on the Node class to allow addition of a single Label to a Node.

This method centralizes the creation of an array with the provided Label, passes it directly to the addLabels method, and returns the list of all Node Labels.

public function addLabel(Label $label)
{
        $labels = array($label);
        return $this->addLabels($labels);
}

[stevenmaguire]

This build is failing and it appears to be related to https://github.com/php-memcached-dev/php-memcached/issues/126

A quick review of Travis CI indicates that this issue is also affecting PRs #121 and #117

[jadell]

Do we need to extend the Node class's API for this? Would it be enough for the existing addLabels on the Node (or better yet, the Client) class to check using is_array and wrap the parameter if it isn't already an array? Same questions apply to #167

[stevenmaguire]

@jadell I would lean on you for final say. For me from a style perspective I don't think these solutions are too much overhead when compared against the value of having method names the describe the functionality.