Allow single user access by login name similar to team authorization.

This is backwards compatible if GITHUB_USER_NAME is not set.

Access will fail if (either GITHUB_TEAM_ID or GITHUB_USER_NAME is set) and (the user is neither the named one nor on the named team.)
Rename GITHUB_USER_NAME -> GITHUB_LOGIN.
Merge pull request #30 from phs/phs/single-user-access

Allow single user access by login name similar to team authorization.