Replace icon for `add_to_calendar` action with nicer graphic.

+ Add `_dark` suffix to file name for being prepared for scheme switch.

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I replaced the icon for the **calendar action** since I do not think it is very intuitive. I  created a custom icon based on the [Font Awesome calendar icon](http://fortawesome.github.io/Font-Awesome/icon/calendar/). I have the icon variant for a light app theme available in case you wanna implement switchable themes somewhen in the future.

**Before** 

![schedule-before](https://f.cloud.github.com/assets/144518/1781606/89f3ebba-6898-11e3-9d15-c5e33b06ddc2.png)

**After**
 
![schedule-after](https://f.cloud.github.com/assets/144518/1781605/89f0587e-6898-11e3-88bd-ee838eaa1442.png)