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).
Yep, good point. I've assigned common.ServerError(err) to result and 
continued loop.

> In state/apiserver/usermanager/usermanager.go:
>
> > @@ -109,3 +110,38 @@ func (api *UserManagerAPI) RemoveUser(args params.Entities) (params.ErrorResults
> >  	}
> >  	return result, nil
> >  }
> > +
> > +// UserInfo returns information on the users in args.Users.
> > +func (api *UserManagerAPI) UserInfo(args params.Entities) (params.UserInfoResults, error) {
> > +	results := params.UserInfoResults{
> > +		Results: make([]params.UserInfoResult, len(args.Entities)),
> > +	}
> > +
> > +	for i, userArg := range args.Entities {
> > +		_, username, err := names.ParseTag(userArg.Tag, names.UserTagKind)
> > +		if err != nil {
> > +			return params.UserInfoResults{}, err
>
> Why bork the whole request for one unexpected param?
>
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> <https://github.com/juju/juju/pull/22/files#r13637082>.
>