there's no real need to use pointers here

This includes tests for relevant methods. Fixes #14.

golint doesn't like the underscore in "url_"

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

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

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.