Support CHECK constraints in migrations and schema.rb

As mentioned at the bottom of [this RailsGuide section](http://edgeguides.rubyonrails.org/active_record_migrations.html#reverting-previous-migrations), CHECK constraints cannot be expressed in `schema.rb`. Please support them.

A syntax suggestion:

``` ruby
    add_check 'my_table', 'a < b', name: 'a_less_than_b'
    remove_check 'my_table', 'a_less_than_b'
```
