Add user info cmd

Add the`$juju user info` cmd to call the recently landed UserInfo API
method. See user docs in user_info.go for an outline of how `juju
info` behaves.
Merge pull request #126 from waigani/user-info-cmd

Add user info cmd

Add the`$juju user info` cmd to call the recently landed UserInfo API method. See user docs in user_info.go for an outline of how `juju info` behaves.