Add STATIC column support

Allows you to have static columns when using a clustering key.

e.g.

    class Post
      include Cequel::Record

      key :blog_subdomain, :text
      key :id, :timeuuid, auto: true
      column :blog_title, :text, static: true
      column :title, :text
      column :body, :text
      column :created_at, :timestamp
    end