Add ActiveDirectory membership validator
No depth argument for this strategy
Search directly against directory, simplify match check

Using GitHub::Ldap::Domain#search forces the base to the domain base
(unsurprising yet surprising).
Add simple, stubbed ActiveDirectory membership validator tests
Short circuit membership valiation without groups to check

Default to true since no groups to validate against translates to
existing handling: if no groups to check against, then they are valid
members.
Stub search results directly h/t @jch
posixGroups are not a thing in ActiveDirectory
Merge pull request #52 from github/ad-membership-valiator

Add ActiveDirectory membership validator