i18n sponsor fields

Feedback from PyConJP development

```
commit 5973e32ebdc231b209b5c058664e8b2b4a1dbc54
Author: MURAOKA Yusuke <yusuke@jbking.org>
Date: Mon Mar 31 15:35:40 2014 +0900

introduce Benefit.content_type which is used to display localized
text
```

Signed-off-by: Hiroshi Miura <miurahr@linux.com>