Implement proof-of-concept of minimum chain of trust updates
Add repository & integration example to assist in the path_hash_prefix implementation
Update the unit tests affected by the design change to log.py
Update system tests following design change to log.py
Merge branch 'develop' into path_hash_prefix
Merge branch 'develop' into chain-of-trust

Conflicts:
	tuf/log.py
Merge branch 'chain-of-trust' into path_hash_prefix
Import chain-of-trust function from @vladdd.