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.

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.

Also properly return a 204 response on methods that don't include a
response body.  This has no actual effect on current tests, but more
accurately reflects what the GitHub API really returns and might make a
difference in the future.

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

- 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.