Draft for a ResourceHook feature.

Hi, me again !

When working, I wanted to be able to trigger a celery task (another use case can be sending a message to websocket server) each time a user create a new resource.

So I tried to implement a basic ResourceHook (a ItemHook can also be implemented), the hook is enabled in <code>settings.py</code>.

Here is how my <code>settings.py</code> looks like:

``` python
from eve.utils import ResourceHook

class MyresourceHook(ResourceHook):
    def get(self):
        print "myresource get hook"

    def post(self):
        print "New myresource", self.document_id


myresource = {
    'item_title': 'myresource',
    'auth_username_field': 'user',
    'resource_hook': MyresourceHook,
...
```

Tell me what you think about my implementation and about the idea.

Thanks !