Add markup and js for platform notifications #3951

#3951 

#### Changes

* Implements `hook_preprocess_toolbar()` to conditionally add Notifications markup
* Adds new javascript file for notifications behaviors
* Uses `#attached` to add javascript only when needed

![screen shot 2013-10-09 at 4 54 01 pm](https://f.cloud.github.com/assets/228733/1301580/f8d6a41a-3124-11e3-80af-ee2bd9823512.png)