update example.php with namespace changes: HipChat\HipChat_Exception

The new version catches exceptions correctly.
The old version could result in "Fatal error: Uncaught exception 'HipChat\HipChat_Exception'".
You can verify this by using a Notification token (which has insufficient privileges), instead of an Admin token.
Merge pull request #14 from dxjones/master

update example.php with namespace changes: HipChat\HipChat_Exception