Support for collections as outputs

This is dependent on https://github.com/nelmio/NelmioApiDocBundle/pull/451

```
@ApiDoc(
     output = "array<FooBundle\User>"
)

or 

@ApiDoc(
    output = { "class" = "FooBundle\User", "collection" = true }
)
```

would produce:

```
[] - array of objects (User)

[][first_name] - string
[][last_name] - string

```

```
@ApiDoc(
     output = "array<FooBundle\User> as users"
)

or

@ApiDoc(
    output = { "class" = "FooBundle\User", "collection" = true, "collectionName" = "users" }
)
```

would produce:

```
users[] - array of objects (User)
users[][first_name] - string
users[][last_name] - string
```

I updated the tests already, but will write more to cover the named collection cases.