Add zpop example.
Improve zpop example.
Improve documentation.
Add server close test.
Improve Conn.Err documentation.
Return underlying network errors in Dial methods; improve doc.
Handle temporary errors from underlying net conn.

Update the code to close the underlying network connection on any error
returned from the network connection. This ensures that the Redigo
connection will return errors on subsequent calls to the Redigo
connection. It will also unblock a reader when pipelines are used.