Adds setnx support and some accompanying tests.

- Adds a function to support mocking setnx
- Adds tests for setnx
- Updates README with setnx entry
Please could you also add a test with 'expire', like this :
->integer($redisMock->expire('test-setnx',1))
                ->isEqualTo(1);
        sleep(2);
        $this->assert
           ->integer($redisMock->setnx('test-setnx', 'lala'))
                  ->isEqualTo(1)
Thanks in advance