Address Issues #165, #158, and #147.

Issue 147: Finalize conversion of all written metadata behavior.  This commit ensures that compressed and uncompressed metadata is also written as outlined in the issue.

Issue 158: As requested, updater.refresh() may now unsafely fetch (i.e., unknown file size and hash) Root metadata if valid top-level metadata cannot be downloaded successfully (e.g., top-level keys may have been revoked).  The repository must also sign the new Root file (at least until all clients have updated) with any revoked keys so that clients may successfully update.  After unsafely updating Root, the top-level metadata is updated again as normal (and only once to avoid an infinite loop).  By default, refresh() unsafely updates Root if only invalid top-level metadata can be downloaded, although this behavior may be overriden by the caller if they wish.  Changed default behavior: refresh(self, unsafely_update_root_if_necessary=True)

Issue 165: Delegated roles are no longer added as attributes of a Targets object by libtuf.py (e.g., repository.targets.delegated_role).  The previous bahavior restricted rolenames to Python identifiers (i.e., can only include letters, numbers, the underscore character, and must start with a nonnumeric character).  Now, delegated roles may be referenced as strings (e.g., repository.targets('recently-claimed')) and include characters other than '_'.  In addition, methods have been added to return all the delegated rolesnames of a target (e.g., repository.targets.get_delegated_rolenames()) and the immediate delegated Target objects of a role.  Previous behavior: repository.targets.unclaimed.django.version = 8
Current behavior: repository.targets('unclaimed')('django').version = 8.