Remove dependency on undefined model 'Account'

There is code that references `Account`, but that model is not defined.  This
code change preserves the original functionality if that model is defined, but
allows broader use of the Gem with applications that do not have an `Account`.
Probably just being picky here....but do you think this looks better:

```
if defined? Account
  Account.find_by_event_code(event_code).slug
else
  ''
end
```