Add HMGET support and WITHSCORES support for ZRANGE

Added HMGET support, and added WITHSCORES support for ZRANGE, with tests and readme update.

I also refactored the ZRANGEBYSCORE code a bit so that ZRANGEBYSCORE and ZREVRANGEBYSCORE could share code (sorry for the messy diff).
Please could you rather test here `hmget` with `expire`, with something like this :
```php
->integer($redisMock->expire('test', 1))
->isEqualTo(1);
sleep(2);
$this->assert
->array($redisMock->hmget('test', array('raoul', 'test1')))
->isEqualTo(array(
'raoul' => null,
'test1' => null
));
```
Thanks :smiley: