Introduce a context object on RESTStorage to prepare for future changes

Per feedback on #1417, I have added a Context object to the major apiserver interfaces.

This is a prerequisite to staging in support for enabling the namespace of a request to get set on a context and passed down the call-stack.

Other future potential use cases:

1. request id (traceability)
2. oauth token for request (when user is calling an API that delegates to another call)
3. user unique identifier (per accts, so actions can get traced to a user)
4. etc.

If folks are ok with this general pattern, I will add the Godeps changes in an additional commit.

@lavalamp @smarterclayton