Add membership_validator config option, default to detect
Add Detect membership validation strategy

Detects ActiveDirectory capabilities, defaults to Recursive.
Move strategies map to GitHub::Ldap::MembershipValidators
Update required capability OID to match 2008 R2 v61
Proactively handle capapbility query errors, instrument
Fix detect_strategy doco
Ensure detect_strategy returns the strategy class
s/if/whether/
Move capabilities lookup to GitHub::Ldap
Only instrument when we query for capabilities
Server returns supportedCapabilities
Test Detect meta-strategy
fix typo
add test for validator detection
Merge remote-tracking branch 'origin/master' into default-membership-validator
additional docs on AD membership validator
Merge remote-tracking branch 'origin/master' into default-membership-validator
Merge pull request #58 from github/default-membership-validator

Detect appropriate membership validator strategy by default