Add query filter support for map-reduce

Example:

```python
from djangoappengine.mapreduce import pipeline

class PipelineBase(base_handler.PipelineBase):

    def run(self):
        yield pipeline.DjangoModelMapreduce(models.Comment, mapper, reducer,
            extra_mapper_params = {
                'filters': [
                    ('user_id', '=', 123),
                    ('some_value__isnull', '=', True),
                ]
            }
        )
```