Adds db install / uninstall

#fixes #50 

Adds `message_broker_producer` database table.

**To test**
- Disable -> uninstall / re-install module.
- Check for `message_broker_producer` table in database.
  - `vagrant@dev:/var/www/vagrant/html$ drush sql-cli`
  - `use dosomething`
  - `show tables`
  - `explain message_broker_producer`
- Disable -> uninstall again to confirm database is removed.

**Note**: Your local vagrant database installation will have to be rebuilt after testing as all of the message_broker_producer variable settings will be removed in the uninstall step.

**Double Note**: As suggested by @aaronschachter , this pull request will need to be tested as an update rather than in uninstall/install to confirm support of the upgrade path that will be used by the DS affiliate sites.
  - `vagrant@dev:/var/www/vagrant/html$ drush updb`

Related:
#32 
Hmm, ok. I can dig it. We just don't do this anywhere within the dosomething repo... mainly because we're careful about checking for fields existing already, etc, or going back in and modifying update hooks (by commenting them out) that shouldn't be run for new installations anymore.

I'd say the argument against doing it this way is just that there's duplication of schema.  But as long as everything matches, go for it.