Set the daemon status when shutting down

[keepsimple1]

This is to address issue #148 :

• Introduce a new enum DaemonStatus.
• shutdown() returns DaemonStatus::Shutdown in the success case. No more commands can be accepted after.
• The underlying channel is guaranteed to be closed when DaemonStatus::Shutdown status returned.
• A new API status() to pull the current daemon status.

This is a breaking change for the shutdown() API as its return type changed:

pub fn shutdown(&self) -> Result<Receiver<DaemonStatus>>