Add option for clearing org before harvesting

Before (a foo dataset and a remaining DCAT dataset)

![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
```

After (just the DCAT harvested datasets)

![after](https://cloud.githubusercontent.com/assets/705860/5387565/a85f7dea-80a1-11e4-8da8-7c05006972db.png)