Project ical box on project overview

![screenshot from 2014-01-29 15 49 13](https://f.cloud.github.com/assets/2074433/2030282/97a8231c-88f4-11e3-8728-e7a41cbb3961.png)

This creates a box to download the iCal file on project overview page. The url makes possible to export ical to Google Calendar and other calendar managers.