Analytics Deferred Queries for Go

Per sdk-rfc #45, add a feature to set up and execute a deferred query on the analytics service. This will allow asynchronous execution of an Analytics query with the ability to retrieve the results at a later date or poll for the results until complete.

Do take care to abstract away any of the details of the Analytics service REST interface, as it will likely change in the future. As an example, it may make sense to have the user API treat handles as opaque rather than expose it as a URI.

Label the API as 'volatile' per our interface stability.