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've rejigged params/api interface to work as a bulk api call as 
mentioned earlier. I'm happy to change it to your suggestion below if 
needed.
>
> In state/api/params/params.go:
>
> > @@ -741,3 +741,16 @@ type EnsureAvailability struct {
> >  	// If this is empty, then the environment's default series is used.
> >  	Series string
> >  }
> > +
> > +// UserInfo contains arguments for the UserInfo client API call.
> > +type UserInfo struct {
> > +	Username string
> > +}
> > +
> > +// UserInfoResult holds the result of a UserInfo call.
> > +type UserInfoResult struct {
> > +	Username    string `yaml:user-name json:user-name`
> > +	DisplayName string `yaml:display-name json:display-name`
> > +	ID          string `yaml:id json:id,omitempty`
>
> Not quite sure about the layering here. We'd generally have something 
> like:
>
> type UserInfo struct {
> Username string |json:"user-name"|
> DisplayName string ...
> ID string ... // although: what will we be using this for? no point 
> showing it to the user if it doesn't help them interact with the 
> system at all...
> }
>
> type UserInfoResult struct {
> Result *UserInfo
> Error *Error
> }
>
> type UserInfoResults struct {
> Results []UserInfoResult
> }
>
> —
> Reply to this email directly or view it on GitHub 
> <https://github.com/juju/juju/pull/22/files#r13437406>.
>