Expose transport args to users
Add a most basic async test
Add a tag role for frontends and make the blocking one implement it
HasRiak can refer to any kind of frontend

Making sure it's the kind of frontend you can work with is up to you.
Add asynchronous request classes

They're identical to the synchronous ones, but also carry a result callback with
them.
Add a first version of an async http transport
Make the async frontend use the async http transport