RFC: JSON-LD

Please let me know your thought on these JSON-LD exporter helpers. Some findings:

* We need to standardize on UTC.
* If you plan on exporting your model via JSON-LD it is better if you define it in a standard way (context, [example](http://www.openannotation.org/spec/core/publishing.html)) from the beginning; otherwise you will need to implement translations.
* We need to think of a way of specifying a default context for SmartJson models.
* It is quite tricky to send JSON search queries via plain GET, as we will receive all parameters as strings and thus we cannot take advantage of, for example, Mongo native types.