Foreign Key request

Hi!

I added Foreign Key creation, some examples:

    migration 1, :create_people_table do
      up do
        create_table :jobs do
          column :id,   Integer, :serial => true
          column :name, String
        end
        create_table :people do
          column :id,   Integer, :serial => true
          column :desc, String
          column :job_id
        end

        modify_table :people do
          add_foreign_key :job_id, :jobs
        end
      end

      down do
        modify_table :people do
          drop_foreign_key :jobs
        end
      end
    end