Add option for clearing org before harvesting

Suppose we want to harvest the following DCAT:

```json
{
    "title": "Catálogo de datos abiertos de Ayuntamiento de San Luis Potosí",
    "dataset": [
        {
            "title": "Licencias Comerciales Autorizadas por giro de San Luis Potosí"
        },
        {
            "title": "Reportes de Atención Ciudadana de San Luis Potosí"
        }
    ]
}
```

And that org has 2 datasets: a foo and one match (with the DCAT catalog):

![before](https://cloud.githubusercontent.com/assets/705860/5387563/a4b1c2c0-80a1-11e4-989f-9d4d9fc65ca9.png)

```zsh
(ckan)➜  ckanops git:(purge-and-harvest) ✗ python ckanops.py --purge-harvest http://adela.datos.gob.mx/ayuntamiento-de-san-luis-potosi/catalogo.json
```

You can now clear the org's datasets before harvesting

![after](https://cloud.githubusercontent.com/assets/705860/5387565/a85f7dea-80a1-11e4-8da8-7c05006972db.png)


Fixes: #12