Users.List renamed to Users.ListAll to align with the forthcoming
Repositories.ListAll method.

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

- 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.