Add function User->inGroupNamed($groupName)

I've gotten really tired of writing 
````
$employeeGroup = Sentry::getGroupProvider()->findByName('Employee');
if ( $user->inGroup($employeeGroup)) {
...
````
Adding one little function lets me do this:
````
if ( $user->inGroupNamed('Employee)) {
...
````

Not only is it (I think) easier to read, it saves a DB call that we don't really need.