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).
Tag annotation removed.
>
> In state/api/params/params.go:
>
> >  	// Tag is here purely for backwards compatability. Older clients will
> >  	// attempt to use the EntityPassword structure, so we need a Tag here
> >  	// (which will be treated as Username)
> > -	Tag         string
> > -	Username    string
> > -	DisplayName string
> > -	Password    string
> > +	Tag         string `json:tag,omitempty`
>
> NO... we can't add the new serialisation stuff here. The whole point 
> of having Tag as a capital is to make sure it works.
>
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> <https://github.com/juju/juju/pull/22/files#r13686098>.
>