Add a second scheduler code in Event import

Add a `HAVE_CODE2` in `RDEvent` to manage a selection with two scheduler codes. When the second scheduler code is defined, only carts with the two selected codes are selected. The second scheduler code rule can be broken if needed.

![rivendell-scheduler-code-2-draft-detail](https://cloud.githubusercontent.com/assets/24570/4463789/34f971c2-48ce-11e4-8d43-e206a6be7a1d.png)