Using the search function with the cursor api returns the same results continually

Rather than:

```
    if hasattr(function, 'iter_metadata'):
        since_id = content[function.iter_metadata].get('since_id_str')
```

it should be 

```
    if hasattr(function, 'iter_metadata'):
        since_id = content[function.iter_metadata].get('max_id_str')
```

Currently, the cursor function only goes forward in time so since_id should be set to maximum id of the latest tweets. 
