adds CheckMemberhsip, CheckPublicMembership, and RemoveMember

this is particularly important for methods like
Organizations.CheckMembership which return a 404 to indicate a
successful response of "false".

also minor doc cleanup

includes Users.Get, Users.Edit, Users.List

- standardize on "url_" for internal vars rather than "urls" or "url"
- move var definition closer to first use in several tests
- add .gitignore for test binary

This commit adds the core library funcationality and establishes the
general calling style and testing structure.  Only a few GitHub API
methods related to organizations and repositories are included in this
first commit, mainly to cement the calling style.