update Membership to keep up with latest API changes

Latest blog post:
https://developer.github.com/changes/2014-08-28-accepting-organization-invitations-from-the-api/

- TeamMembership has been renamed to Membership, since it now covers
  organization membership as well
- Status field renamed to State
- added new methods: ListOrgMemberships, GetOrgMembership,
  EditOrgMembership