add site_name helper method for link model.
add site_name helper method to server-side link model and getSiteName helper method to backbone model.
clean up model to_json specs.
use serializable_hash not as_json so that associated models preserve options set here.
set embedly api key from env.
fix VCR configuration to ignore API key, and turn VCR off by default, only turn on inside use_cassette block.