Added method domains.vanitynameservers
Added memberships section
Added method domains.memberships.list
Added method domains.memberships.add
Added method domains.memberships.delete
Removed string input on toggle methods

i.e. domains.autorenew( domain, 'on', cb )
-> domains.autorenew( domain, true, cb )
Added method domains.whoisPrivacy
Improved error handling based on statusCode