Added support for MPD's "idle" & "noidle" commands.

Hey,

i've added 

- ```func (c *Client) Idle(subsystems ...string) ([]string, error)```
- ```func (c *Client) NoIdle() error```
- ```func (c *Client) readList(key string) ([]string, error)```

The obligatory spec: http://www.musicpd.org/doc/protocol/ch03.html#command_idle

I think this is the last pull request. I have now everything i need - i think.

Thanks for the review.