Add test repository setup and teardown helpers.

Added test_repo test helper method to generate a unique-enough
repository name and filter it from VCR cassettes as
<GITHUB_TEST_REPOSITORY>.

Added setup_test_repo test helper method to create a new repository on
GitHub to test against. Method returns the newly created repository.

Added teardown_test_repo test helper method to delete a test repository
from GitHub. Method takes a full repository name.
Add VCR cassette placeholders for test repository