Return column config, and store config/data for ExtJS ColumnModel and Store

  Examples:

    # app/controllers/backend/debtors_controller.rb
    def index
      @column_store = column_store_for Debtor do |cm|
        cm.add :id
        cm.add "full_name_or_company.upcase",   "Full Name",      :sortable => true, :dataIndex => :company
        cm.add :surname  # Header will be autogenerated
        cm.add :email,                          "Email",          :sortable => true
        cm.add :piva,                           "Piva",           :sortable => true
        cm.add :created_at,                     "Creato il",      :sortable => true, :renderer => :date, :align => :right
        cm.add :updated_at,                     "Aggiornato il",  :sortable => true, :renderer => :datetime, :align => :right
      end

      respond_to do |format|
        format.js
        format.json do
          render :json => @column_store.store_data(params)

          # or you can manually do:
            # debtors           = Debtor.search(params)
            # debtors_count     = debtors.size
            # debtors_paginated = debtors.paginate(params)
            # render :json => { :results => @column_store.store_data_from(debtors_paginated), :count => debtors_count }
        end
      end
    end

    # app/views/backend/index.rjs
    page.grid do |grid|
      grid.id "debtors-grid" # If you don't set this columns are not saved in cookies
      grid.title "List al debtors"
      grid.base_path "/backend/debtors"
      grid.forgery_protection_token request_forgery_protection_token
      grid.authenticity_token form_authenticity_token
      grid.tbar  :default
      grid.store do |store|
        store.url "/backend/debtors.json"
        store.fields @column_store.store_fields
      end
      grid.columns do |columns|
        columns.fields @column_store.column_fields
      end
      grid.bbar  :store => grid.get_store, :pageSize => params[:limit] # Remember to add after defining store!
    end