Merge remote-tracking branch 'origin/master' into log-to-console
Revert "Print, and log, messages in command-line utilities."
Endless attack test fix
Endless attack test fix
modified update.py
Fixed various tests.
endless_attack_test_fix
endless_attack_test_fix
Tests no longer run automatically when imported.
aggregate_tests now loads all the unit tests into one suite and runs them together,
so that any failures and errors show up together in a concise report.
Fix the endless data attack issue

modification of updater.py for download.py

modification of conf.py for fix

modification of test_download.py for download.py

modification of test_updater.py for download.py

add a new test of endless data attack to metadata timestamp.txt

more readable and fix the endless data attack issue.
Update specification and code to recognize the new 'path_hash_prefix' attribute.
Implement proof-of-concept of minimum chain of trust updates
Adapt Zheng Yuyu's changes.
Minor changes following review
Removed an unsafe edge case, but updater unit tests need to be fixed.

Specifically, we do not intentionally set any file metadata to be None
and then download the file unsafely.

Some of the tuf.client.updater unit tests fail because it was previously
possible to unsafely download metadata for any role. We need to fix
this.
Remove test print statements
Merge branch 'master' into path_hash_prefix
Merge branch 'master' into path_hash_prefix
Improved checking of the "paths" and "path_hash_prefix" attributes.

Removed checking whether "path_hash_prefix" is consistent with the
delegated paths in the delegator, because now the delegated paths may
list directories instead of simply files.
Improved checking of the "paths" and "path_hash_prefix" attributes.

Removed checking whether "path_hash_prefix" is consistent with the
delegated paths in the delegator, because now the delegated paths may
list directories instead of simply files.