Add meta method to JSON API adapter to add meta properties

Don't render meta if empty
Add meta properties to HAL (alias to property)
Add meta properties to Siren (alias to property)
Mention `meta` in README.


This commit was made at the RailsConf 2014. Yay!
Add link to the meta portion of the JSON API website

http://jsonapi.org/format/#document-top-level