Update tuf-spec.txt and implement "lazy bin walk"

tuf-spec.txt was updated to include the latest metadata changes, such as version numbers, and the "lazy bin walk" scheme was implemented in updater.py.
Merge branch 'master' of https://github.com/theupdateframework/tuf
Expand comment and add missing prefix comparison in updater.target()
Modify _ensure_all_targets_allowed() to also work with path_hash_prefix
Interim bugfixes in updating.
Fixed a few bugs and refactored the target workflow.