Conflict resolution strategy

This patch implements conflict resolvers by adding the method ``Riak\Output\Output#getObject``

If the `Riak\Output\Output` contain more than one object/siblings 
it will try to resolve its siblings using a  `Riak\Output\ConflictResolver` specified by key or bucket.

The ConflictResolver will be responsible for resolving the siblings into a single object.
if there are no resolver configured it will throws a `Riak\Exception\NonUniqueException`


conflict resolver impl:
```php
<?php

class MyResolver implements \Riak\Output\ConflictResolver 
{
    public function resolve(\Riak\ObjectList $objects)
    {
        $winner = $objects->first();
        $value  = 0;

        foreach ($objects as $object) {
            $value += intval($object->getContent());
        }

        $winner->setContent($value);

        return $winner;
    }
}
```

Bucket resolution :
```php
<?php
$bucket->setConflictResolver(new MyResolver());

$output = $bucket->get($key);

echo $output->hasSiblings(); // true

$object = $output->getObject();
```


By key resolution :
```php
<?php

$options  = new \Riak\Input\GetResolverInput(new MyResolver());
$output    = $bucket->get($key, $options);

echo $output->hasSiblings(); // true

$object = $output->getObject();
```