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 agree in some senses, but there are two separate classes that depend on the same behavior that didn't feel right duplicating in (the ActiveDirectory MemberSearch and MembershipValidation strategies). It's a mix of AD-specific feature detection with general capability exposure.

I'm not crazy about it, but it unblocks this PR and can be easily changed in the future when a better approach can be considered, particularly because it's internal only. I think this need will diminish with the changes you outline in https://github.com/github/github-ldap/pull/69#discussion_r21327407, so it is likely temporary.

Thoughts?