Create testcase to show intention
Let haveFriend() to receive friend's actor class

This fixes #1386.

Passing an actor class is useful to create a friend with specific role, that is
implemented with step object like this:

    $admin = $I->haveFriend('admin', 'AcceptanceTester\AdminSteps');
    $admin->does(function(AcceptanceTester\AdminSteps $I) {
      $I->approvePost();
      $I->seeNewPostMessage();
    });

The original $I and $admin both can be specialized to their roles.
Add docstring
Merge pull request #1394 from tomykaira/friend_with_steps

Friend with steps