Featured content

`streamhub-sdk/content/types/livefyre-content` now has two new methods for dealing with Featured Content
* `.isFeatured()` - returns a Boolean of whether the Content is featured
* `.getFeaturedValue()` - returns a Number representing the 'featured value' of the Content. Useful for sorting by the featured value

`streamhub-sdk/collection/featured-contents` is a new Class that represents the Featured Contents in a Collection. It currently has a single notable method
* `createArchive()` - Create a `streamhub-sdk/collection/streams/featured-archive` object, which is a `stream/readable`, that represents the historically featured Content in the Collection. It reads out the featured content from bootstrap init, then goes to featured-all.json for the rest when/if needed
* In the future, we can add a `createUpdater` method that emits newly featured Content, as well as make FeaturedContents a Duplex stream like Collection is.

Add `examples/featured`, which pipes the FeaturedContents archive from a test Collection into a ListView.