Enforce namespace unique constraints

This is the update to actually enable namespace support.

At the moment, I pass a namespace as a query parameter in order to support namespace scoping on the v1beta urls.  Once v1beta3 lands, I will modify to pass namespace in the path.  Note the client libraries were modified to make this easy to do, so should be a trivial change once ready.

I broke the commits into logical sections to improve readability.
Is there a reason why this function takes an api.Context? That is for tracking client requests server side. There is not much sense forcing clients to use it, no? I think this should just take a string. I also think it's something you should set on the client object, not the request object-- if you're doing namespaced operations, you probably want many in a row with the same namespace.

Very sorry for not noticing this earlier, I was out all last week and have just been catching up this week.