Add logout command

"docker logout [SERVER]" will remove the registry server' credentials from
.dockercfg file. If a server is not specified, it will log user out of the
default docker registry server

Docker-DCO-1.1-Signed-off-by: Daniel, Dao Quang Minh <dqminh89@gmail.com> (github: dqminh)

docs/man/docker-logout.1.md

@@ -0,0 +1,24 @@
+% DOCKER(1) Docker User Manuals
+% William Henry
+% APRIL 2014
+# NAME
+docker-logout - Log the user out of a Docker registry server
+
+# SYNOPSIS
+**docker logout** [SERVER]
+
+# DESCRIPTION
+Log the user out of a docker registry server, , if no server is
+specified "https://index.docker.io/v1/" is the default. If you want to
+logout of a private registry you can specify this by adding the server name.
+
+# EXAMPLE
+
+## Logout of a local registry
+
+    # docker logout localhost:8080
+
+# HISTORY
+April 2014, Originally compiled by William Henry (whenry at redhat dot com)
+based on docker.io source material and internal work.
+

docs/man/docker.1.md

@@ -124,6 +124,9 @@ inside it)
 **docker-login(1)**
   Register or Login to a Docker registry server
 
+**docker-logout(1)**
+  Log the user out of a Docker registry server
+
 **docker-logs(1)**
   Fetch the logs of a container