When a record is created, #id returns a string.

This ensures ID is string everywhere:

```ruby
event = Event.create(name: "Ruby Meetup")
event.id #=> "1"

event = Event.find(name: "Ruby Meetup")
event.id # => "1"
```
Merge pull request #145 from frodsan/id_returns_string

When a record is created, #id returns a string.