Add automatic migration mechanism.

Usage:
```ruby
DB.upgrade_table? :accounts do
  primary_key :id
  column :title, String, :default => 'foobar'
end
class Account < Sequel::Model
end
```

The :accounts table will be created if it does not exist. If it exists and lacks :id or :key column, the corresponding column will be added with ALTER TABLE. If the table already has the column, no action will be performed on it.