Add identity method to Migration DSL

This adds an `identity` method so you can create tables:

```ruby
create_table :foos, force: true, primary_key: false do |t|
  t.identity :id

  # rest of table definition
end
```
Merge pull request #15 from jaredonline/patch-1

Add identity method to Migration DSL sorry this has taken so long to get to -- i appreciate your help on this.
i'll merge this and push a new gem.