Feature/add ical output

This PR addresses the last part of the Issue php-ug/php.ug#12. 

It introduces an API that returns a JSON Object containing the next Event of a usergroup or returns an error when no event could be retrieved.

The API can be found at ```http://php.ug/api/v1/usregroups/next-event/<usergroup-shortname>```

It returns the following JSON:
```JSON
{
    "start": "RFC2822-encoded Start-Date",
    "end": "RFC2822-encoded End-Date",
    "location": "Location-String as given in the iCalendar-Event",
    "summary": "The Title of the event",
    "url": "An URL that is associated with this Event",
    "description": "A more in depth description of the event"
}