Merge pull request #226 from vladimir-v-diaz/develop

Add self-signed certificates, simple_https_server.py, and a test case for https downloads
Merge pull request #227 from vladimir-v-diaz/develop

Update test_download.py comments and test case.
Merge pull request #228 from vladimir-v-diaz/develop

Review, update, document, and test #216.
Add test condition for repository_lib.create_tuf_client_directory().

Verify that non-OSError.EEXist exceptions are re-raised.
Add test coverage for repository_lib._log_status_of_top_level_roles().

Verify that status() does not raise 'tuf.InsufficientKeysError' nor 'tuf.UnsignedMetadataError'.