villein v0.5.0

= Changes

- Villein::Agent: Now handle serf events in 1 thread (by default).

  Because originally Serf handles events in 1 thread, and it is a
  design. So we follow it.

  Use parallel_events:true option if you still want to handle events in
  parallel.

  But still queries respond parallel in default. Use async_query:false
  option if you want to make it serial.