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": ISO Encoded Start-Date,
    "end": ISO-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
}