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.

This adds basic methods for working with issues, assignees, and issue
comments.  Issue events, labels, and milestones are not included in this
commit. (Fixes #7)

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.

add methods to get, create, edit, and delete org teams

internally, http.Error uses fmt.Fprintln() which will append a newline
to what should be an empty response body.  In one of my previous tests,
this was causing spurious errors.  I've since rewritten that test, but
still best to avoid it here.

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

adds CheckMemberhsip, CheckPublicMembership, and RemoveMember

also minor doc cleanup

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.