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).
Done. Tests added to params_test.

> 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`
> > +	Error       *Error `yaml:error json:error,omitempty`
>
> I also notice that you have omitempty lines, but you don't test the 
> serialized form to see whether they are present or not. I'm not sure 
> if it matters, but it does seem like something that if we care that 
> they are omitted, we should probably ensure that.
>
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> <https://github.com/juju/juju/pull/22/files#r13422722>.
>