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 

@jch really like these recommendations! This was definitely the kind of feedback I feel was most needed, thanks.

I think these can be tackled in a follow up PR, though, and it would be better to keep this PR focused and consistent with what's already in master before attempting these changes. That sound like a solid approach? Or is there something here that is a blocking concern?