added ZeroClipboard integration

this pr adds integration with zero clipboard[1].

clicking the button copies the exception and its backtrace into the clipboard to ease e.g. creation of bugtracking tickets etc.

![clip](https://cloud.githubusercontent.com/assets/120441/3012369/f39c7b60-df38-11e3-9363-43cd8ccf5dfa.png)

clicking the button copies the following plain text into the clipboard:

```
Exception thrown with message "wah"

Stacktrace:
#4 Exception in XXXXX/app/www/controllers/IndexController.php:12
#3 IndexController:index in XXX/lib/ActionController.php:778
#2 ActionController:invokeAction in XXX/lib/ActionController.php:290
#1 ActionController:render in XXXCore.php:65
#0 include_once in XXX/public/www/index.php:29
```

Since this feature requires a external `ZeroClipboard.swf` file, I adjusted the path to load it from the cloudfare CDN. The "Copy to Clipboard" button only appears when the external resource could be loaded properly.

[1] https://github.com/zeroclipboard/zeroclipboard