Feature/meta (#81)

* Added back the Metable implementation for our example models.

* Examples updated to use jsonapi.Links and jsonapi.Meta types rather than the underlying map[string]interface{}

* Commenting the Metable interface and moving the Meta type beside it.

* Use the headerAccept constant defined for the example app.

* Make the Meta field of a Link of type Meta rather than a map[string]inteface{}

* Convert the map[string]interface to a Meta in the test.

* Updated the readme to include a deeply nested, varying typed meta example.

* Moving all testing models to their own file; preparing to write a more complete Meta test.

* Make the Meta test also check for the value of the detail key. Moved more test support models.

Author: aren55555