JSON schema for JSON-LD documents

This will allow JSON editors to provide statement completion and
tooltips for authors of JSON-LD documents. Here's an example of what that could look like

![jsonld-schema](https://f.cloud.github.com/assets/1258877/2431280/52b8a5ba-ad25-11e3-82a6-d883dde45a43.png)

I validated all the examples of JSON-LD I could find using
http://json-schema-validator.herokuapp.com/