Add  Server API method: UserInfo

Add a method to the UserManagerAPI which takes a username and returns a struct of user information (username and display name at this stage).
I think tags and Entities makes the code obtuse - what does a tag have 
to do with a user? Also, it becomes very hard from reading the code to 
understand that the API is expecting a collection of usernames.

I have made it a bulk API call though. See what you think.


> In state/apiserver/usermanager/usermanager.go:
>
> > @@ -109,3 +109,24 @@ func (api *UserManagerAPI) RemoveUser(args params.Entities) (params.ErrorResults
> >  	}
> >  	return result, nil
> >  }
> > +
> > +// UserInfo returns information on a user with the username in args.Username.
> > +func (api *UserManagerAPI) UserInfo(args params.UserInfo) (result params.UserInfoResult, err error) {
>
> this doesn't look like a bulk API... please accept Entities, 
> containing tags, and return a UserInfoResults
>
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> <https://github.com/juju/juju/pull/22/files#r13437435>.
>