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 

I feel this module is a premature extraction. It's specific to ActiveDirectory, and doesn't feel useful outside of this context.