Since the library has received external contributions, Google is no
longer the sole copyright holder.  Update the boilerplate at the top of
each file to reflect that, and add AUTHORS and CONTRIBUTORS files to
track this information.

there's no real need to use pointers here

This includes tests for relevant methods. Fixes #14.

add methods for testing request method, request form values, and URL
parse error

use temporary variable for expected value to prevent mismatch between
tests and resulting error messages.

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

use temporary variable to prevent mismatch between tests and resulting
error messages.

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

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