Endless attack test fix
Endless attack test fix
modified update.py
endless_attack_test_fix
endless_attack_test_fix
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.
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.
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.
Bugfix.
Add repository & integration example to assist in the path_hash_prefix implementation
Merge branch 'master' into path_hash_prefix
Fix some bugs with tests.
Merge branch 'master' into path_hash_prefix
Merge remote-tracking branch 'upstream/master'
Accommodate including compressed versions of release in timestamp.
unicode fix in python2