Secondary indexes

```ruby
class Post < Cequel::Model::Base
  key :permalink
  column :title, :text
  column :author_id, :uuid, :index => true
end
```

Should set up the secondary index in the schema and expose:

```ruby
Post.with_author_id(author_id) # returns a record set
Post.find_by_author_id(author_id) # returns the first Post with that author_id
Post.find_all_by_author_id(author_id) # returns an array of Posts with that author_id
```