Added vacation feature

Added CRUD, model, views and tests.
Admin can import vacations from Google Calendar.
When user creates, updates or deletes event, it appears in Google Calendar.
Set table view as default on Available users page.

![zrzut ekranu z 2014-09-03 14 06 47](https://cloud.githubusercontent.com/assets/6718923/4133891/8d71772a-3364-11e4-94c9-dd9a2382fb8c.png)
![zrzut ekranu z 2014-08-25 14 34 09](https://cloud.githubusercontent.com/assets/6718923/4029827/a4a20aba-2c55-11e4-9ba2-53d8e0c63e48.png)
![zrzut ekranu z 2014-08-25 14 34 32](https://cloud.githubusercontent.com/assets/6718923/4029838/b39fefb4-2c55-11e4-8cba-59130def6e62.png)
It would be good idea to move this code to partial, eg:
```
if user.vacation.present?
  = render partial 'row'
```