Implement `asc` and `desc` scopes

The `Assorted::Scopes` module has the methods, which get extended into `ActiveRecord::Base`. Pretty simple, really.
Yep, you can definitely supply your own `created_at`:

```ruby
ExampleRecord.create(created_at: 1.day.ago)
# => #<ExampleRecord id: 1, user_id: nil, example_count: nil, created_at: "2014-09-11 12:43:55", updated_at: "2014-09-12 17:43:55">
ExampleRecord.pluck(:created_at)
# => [2014-09-11 17:43:55 UTC]
```