Pass objects in fixtures data

Question | Answer
-------- | ------
Bug fix ? | yes
New feature ? | yes
BC breaks ? | no
Tests pass ? | yes

I've add a little condition in Executor in order to be able to pass objects (like a DateTime) to the object setters.

This is needed when we use [Faker with your FixturesBundle](https://github.com/DavidBadura/FixturesBundle/blob/master/Resources/doc/faker.md)

Example :

``` yaml
users:
    properties:
        class: "MyCompany\Bundle\CoreBundle\Document\User"
    data:
        user{0..2}:
            firstname: <firstName()>
            lastname: <lastName()>
            birthdate: <dateTimeThisDecade()> # Without this patch, the command fails here
```