#enqueue should be blocking on the caller

[bklang]

Most Adhearsion operations are blocking: if you start #play, or #ask (for example) the caller is blocked until the method returns some value. This makes developing easier for a lot of use-cases because you don't have to think about dealing with asynchronous behavior.

Right now, ES's #enqueue method is non-blocking. It should be blocking by default, and the documentation should show how to call the method both sync and async.