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

also add specific error type checking for invalid JSON test

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

this is particularly important for methods like
Organizations.CheckMembership which return a 404 to indicate a
successful response of "false".

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.