Issues/984 party logos

Part of #984 - can close it after merging and deploy. Will need to run this command to add images to the parties:

    ./manage.py core_import_popolo --commit pombola/south_africa/data/south-africa-popolo.json

- add image files to repo and to popolo JSON file
- add images to orgs
- display images for orgs that have them

Note image is rather low res atm. Could fix this by changing the size shown, or finding better images:

![anc____pombola_south_africa](https://f.cloud.github.com/assets/187630/1582875/69d36ebe-51f5-11e3-95d6-2ab1d764e83c.png)