Add GET parameter for specifying the query processor

Fix #188.

Signed-off-by: Don Naegely <naegelyd@gmail.com>
Setting this to `pipeline.query_processors` always results in the user of the `default` QueryProcessor since it appears that choices is set to the QueryProcessor object itself which does not appear to be iterable. From my testing, it appears that we can either set `choices=pipeline.query_processors.processors` which is a dictionary and is, therefore, obviously iterable OR we can update Avocado with the following change:

```python
class QueryProcessors(object):
    ...

    def __iter__(self):
        return self.processors.__iter__()

    ...
```

which will make the QueryProcessor object iterable and make choices defined and make the `is in` operation successful.

@bruth Any preferred approach from the two above?