add last two team membership functions
add functions for managing a team's repositories
test invalid URL on repo creation

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.
add functions for listing and creating repo forks
cleanup tests (no substantive changes)

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