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 yeah, the tasks I outlined below (https://github.com/github/github-ldap/pull/69#issuecomment-65729011) are meant to be done *before* 1.6.0 gets cut. Just wanted to split up the work into separate PRs to keep focus better.