MemberSearch cleanup and polish

As a follow up to #64, this renames `GitHub::Ldap::Members` *member search* strategy to `GitHub::Ldap::MemberSearch` and aims to finish up any significant missing pieces:

- [x] configuration
- [x] Active Directory strategy (followup?)
- [x] detect strategy

cc @jch 

No blocking concern, but I would like to see this done before the freeze for long term maintenance's sake. Your call on where you'd like it done.