allow querying on EmbeddedDocumentFields

Just specify the `__`-separated path in `Resource.filters`:

```python
class FooResource(Resource):
    filters = {
        'bar__baz': [ops.Exact]
    }
```
support specifying filters for EmbeddedDocumentFields

This allows queries like 'document__subkey=value', which otherwise got
confused with operators.
Merge pull request #33 from matchbox/query-embeddeddocumentfield

Query EmbeddedDocumentField