[share api] add OCS api call to set expire date for link shares

Long standing bug https://github.com/owncloud/core/issues/5835 can now be fixes because from OC7 on the expire date is only used for link shares.

This PR add a OCS share api call to set a expire date.

Set expire date:

```
curl -X PUT -u schiesbn:schiesbn http://localhost/owncloud/ocs/v1.php/apps/files_sharing/api/v1/shares/12 -d expireDate="31-07-2014"
```

expire date need to be a well formate date, e.g. 'YYYY-MM-DD' works too.
please add a default value - otherwise it will break the public api - thx