Support multiline text

The following import results in a value without linefeeds:

```
id,product_highlights
gartenholzreiniger,"für alle Hölzer im Außenbereich
auch für Kunststoffmöbel
einfache Anwendung"
```

reader seems to be set incorrectly in [import_view.py](https://github.com/collective/collective.importexport/blob/master/src/collective/importexport/browser/import_view.py#L87-L90) :

```
reader = csv.DictReader(data.splitlines(),
delimiter=",",
dialect="excel",
quotechar='"')
```
See https://docs.python.org/2/library/csv.html#csv.DictReader