Option to use create! instead of create

It would be very convenient to have the option to generate ```create!``` instead of ```create``` statements, so that exceptions stop the seed process.

The pull request adds the 'CREATE_THROWS_EXCEPTION' variable:
```
rake db:seed:dump CREATE_THROWS_EXCEPTION=true
```

See also http://api.rubyonrails.org/classes/ActiveRecord/Validations/ClassMethods.html#method-i-create-21