Main feats:
    + Create purchase order when confirm the requisition
    + Send mail when `notify_purchase` has been clicked
    + Only user in purchase group can convert requisition to purchase
      order
    + Every item in requisition line must be set a supplier before
      converting to purchase order
    + Read-only when requisition is `done` or `cancel`
    + I don't limit min qty from supplier info, if it is needed I will
      add it in future

Notice:
    + You should update `purchase` module due to `purchase.order` has
      been inherited in `requisition` module
    + Mail function has not been tested yet