FEATURE: New "Render Performance" menu item to profile rendering speeds

This pull request adds a new "Render Performance" menu  item to the ember extension. When a user selects the item, they are given a list of the most renderings and how long they took. If there is nested information available (such as a view rendering a list of items) you can expand and view the sub rendering times.

![render-speed](https://f.cloud.github.com/assets/17538/2519800/a4ae1b74-b479-11e3-8d2a-fa84cf57ca8a.png)

This is my first pull request to ember-extension so I am more than willing to change anything around to make sure it gels with the core project. Please give me any feedback, I'll take it happily!