Error serializing field duration = models.DurationField()?

Hi there, i'm not 100% sure if this is a bug or not (I'm newbee here) but i got a problem while querying a Node that uses the field type models.DurationField ( it really returns datetime.timedelta).

```
Traceback (most recent call last):
File "/tmp/leaky_venv/lib/python3.6/site-packages/promise/promise.py", line 65, in try_catch
return (handler(*args, **kwargs), None)
File "/tmp/leaky_venv/lib/python3.6/site-packages/graphql/execution/executor.py", line 375, in
resolved
File "/tmp/leaky_venv/lib/python3.6/site-packages/graphql/execution/executor.py", line 399, in complete_value
return complete_leaf_value(return_type, result)
File "/tmp/leaky_venv/lib/python3.6/site-packages/graphql/execution/executor.py", line 440, in complete_leaf_value
return return_type.serialize(result)
File "/tmp/leaky_venv/lib/python3.6/site-packages/graphql/type/scalars.py", line 51, in coerce_float
return float(value)
TypeError: float() argument must be a string or a number, not 'datetime.timedelta'
```

This error points to line 51 but i think the class GraphQLScalarType (on line 65) is also related.

Here is my env's prop
```
PostgreSQL 10.0 on x86_64-pc-linux-gnu (xubuntu 16.04), compiled by gcc (Debian 6.3.0-18) 6.3.0 20170516, 64-bit

Python == 3.6.1

Django==1.11.6
django-filter==1.1.0
graphene==2.0
graphene-django==2.0.0
graphql-core==2.0
graphql-relay==0.4.5
```
How to reproduce?

1- Create your model
```
class Asset(models.Model):
duration = models.DurationField(null=True, blank=True, help_text="Duration of a given offer/product (auto calculated)")

class Meta:
ordering = ["id"]
```

2-Create your schema
```
class AssetNode(DjangoObjectType):

class Meta:
model = Asset
interfaces = (relay.Node, )
```
3- insert a not-null value on the duration field... and run your query.

```
{assets(first:20,) {
edges {
node {
id
duration
}
}
}}
```

and you should que the error:
```
{
"message": "float() argument must be a string or a number, not 'datetime.timedelta'"
}
```

Commencez par exécuter le Asset model, le AssetNode schema et la duration query fournis sur l’environnement indiqué afin de reproduire l’erreur. Inspectez graphql/type/scalars.py autour de la ligne 51 et GraphQLScalarType autour de la ligne 65, puis vérifiez que l’interrogation d’un Django DurationField non nul s’achève sans l’erreur de sérialisation datetime.timedelta signalée.

Rédigé par le modèle d'indexation à partir du texte de l'issue.