internally, http.Error uses fmt.Fprintln() which will append a newline
to what should be an empty response body.  In one of my previous tests,
this was causing spurious errors.  I've since rewritten that test, but
still best to avoid it here.

This now aligns with the (pretty strong) golang convention of the
package name matching the path basename

adds CheckMemberhsip, CheckPublicMembership, and RemoveMember

also minor doc cleanup

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.