Simplify Search Strategy configuration, detection, default behavior

Removes the `Detect` meta-strategies for the *Member Search* and *Membership Validation* behaviors. Inlines the configuration and detection logic in `GitHub::Ldap` instantiation/configuration, simplifying things drastically. Combines the separate config options into `search_strategy`.

This is to follow up on #69, specifically addressing concerns @jch brought up, summarized here: https://github.com/github/github-ldap/pull/69#issuecomment-65729011

cc @jch @github/ldap