Fixes: #266 - Handle nested serializers in create and update API methods

[bctiemann]

Fixes: #266

Implements create() and update() methods for the synthetic serializer class created by get_serializer_class. This avoids the guard in stock DRF that prevents nested serializers from being automatically generated for M2M fields, and allows the API to be used to create and update Custom Objects with multi-object fields whose values are specified in the payload.

POST: http://127.0.0.1:8000/api/plugins/custom-objects/foo1s/

{
    "foo2s": [1, 2],
    "foo2": 1
}

[arthanson]

Can you add some tests for these, we should probably start doing that for any new additions.