Hide unstable items from the API docs by default

This adds an "unstable" class to anything not marked with `@api stable`.  By default, these items are hidden in the API documentation (people should be discouraged from using them).  A checkbox allows for toggling the display of unstable items.

Default view:
![stable](https://cloud.githubusercontent.com/assets/41094/4018779/f092b1a2-2a54-11e4-8db4-a7bf52b348e9.png)

With "Stable Only" unchecked:
![unstable](https://cloud.githubusercontent.com/assets/41094/4018780/fb516214-2a54-11e4-95a0-d335fb8d5889.png)

This should provide good motivation for making sure we have enough marked stable before the release.