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

golint doesn't like the underscore in "url_"

add test to check for required fields, as well as test for default vs
custom transport.

This allows for making unauthenticated requests subject to your
application's rate limit (as opposed to the lower rate limit for
anonymous requests)

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.

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.

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

if we have an error reading the body of an API error response, still
return an ErrorResposne with the original response struct.  Simply
returning a string stating that we couldn't read the response body
doesn't give the caller anything actionable to work with or attempt to
process.

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.

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