Filter statistics announcements by organisation

The statistics announcements page in admin now defaults to showing the current user's organisation's announcements. The filter allows other organisations to be selected, as well as all organisations.

Story: https://www.agileplannerapp.com/boards/173808/cards/5478

- [x] code review
- [x] PO acceptance

![screen shot 2014-08-06 at 15 30 26](https://cloud.githubusercontent.com/assets/3687/3828245/1d252632-1d76-11e4-89f1-cb57da4c0632.png)