Refactor proposals

Based on migrating the database from last year's site, we discovered that our current strategy to resolve South migration [ignores](https://github.com/pyohio/pyohio/pull/6) is not tenable.

This PR refactors ```pyohio_proposals``` to ```proposals``` and utilizes ```SOUTH_MIGRATION_MODULES``` to override the default symposion migrations for ```proposals```

**Notes**

If you have legacy data, you'll need to truncate the presentations. Considering this affects talks from last year, this should not be an issue :dango: 

```
psql
\c pyohio2014

TRUNCATE TABLE schedule_presentation CASCADE;
```
You will run migrations:  ```python manage.py migrate proposals```