Remove list.sort() and cleanup try-except blocks in signercli.py

Previously, _make_delegated_metadata() attempted to minimize the number of target directories in the "paths" field
of delegations by calculating common root-most directories.  This bahavior was found to be unsafe and removed, and as a result,
the sort of delegated targets is no longer needed.
Update updater.py to retrieve compressed versions of Targets metadata

This change addresses issue #85.  The previous implementation only recognized compressed versions of "release.txt".
Preliminary update preceding the major path_hash_prefix changes

This update addresses issue #86.  It begins by removing the wholesale downloading of all targets metadata and only downloads & verifies the metadata for the roles it only needs; the "lazy walk" scheme.
Merge branch 'master' into path_hash_prefix