Add the has_member and has_group association helpers to specify model class name

[joelvh]

We wanted to create a users association with a CustomUser class. We added the option to use has_member :users, class_name: 'CustomUser', so you can specify the class for specific associations. This is backwards compatible with just passing in association names to has_members.

I think that the only change that's missing from Mongoid is default_members: true option for has_member

[dwbutler]

I understand the need to customize the associations generated by Groupify.

In this case, I think it will ultimately be simpler and more useful to have a separate has_member method which takes an association name and a hash of association options.

has_member :users, class_name: 'CustomUserClass'
has_member :admins, class_name: 'CustomAdminClass'

[joelvh]

@dwbutler OK, I will refactor with has_member method and take a look at the tests that are failing.

[joelvh]

@dwbutler updated with has_member method. I also noticed that there was a missing association_name argument to the associate_member_class method in Mongoid, which is now added....

[dwbutler]

Related: #47

[joelvh]

@dwbutler closing this in favor of #61, which incorporates this addition.